v0.9.2: files the agent delivers come back as cards, and the saved key is used

A turn could write a report and say "here it is" -- and there was nothing to
open. The engine had no way to hand a finished file to the person, and the
front end had no way to show one. This release adds the whole path: a tool to
deliver, a store that keeps the copy, a card in the transcript, and open /
share through the platform. It also fixes two bugs the phone exposed while it
was being tried there.

deliver_file (engine)
- cst_attachment_t.for_user marks a copy that belongs to the person, not to
  the model; cst_attach_import_artifact() stores any bytes up to
  CST_ARTIFACT_MAX_BYTES (64 MB) with the mime taken from the name's
  extension. Attachments are unchanged (content decides the type, 16 MB).
- It is a read-policy engine tool: path resolves inside the workspace, a
  directory is refused, title (optional) is the display name.
- The wire is what keeps it out of the conversation: parts_add_attachments()
  skips for_user entries without even a "not sent" note, and a turn that only
  delivered a file no longer invents an empty user message. tests/test_artifacts.c
  pins that down end to end, including a session-file round trip.

The card (Qt front end)
- filesText() replaced imagesText(): one line per file -- kind, stored name,
  name, origin, mime, bytes, pixels -- decoded by ImageStrip.qml. A for_user
  entry is a file card (extension badge, name, type and size, Open, Share);
  a picture stays a thumbnail and opens on tap. Both shapes are read, so
  sessions written by an older version still draw.
- AgentCore::openArtifact() / shareArtifact() accept a name only when one of
  the session's own messages carries it, and hand the display name to the
  platform layer for the status line.
- Android opens through the app's FileProvider (content:// + ACTION_VIEW,
  resolveActivity() checked, read permission granted) and shares through the
  share sheet; a desktop uses QDesktopServices / the clipboard.
  CST_APP_NO_OPEN=1 makes --uitest run without opening anything.

Two bugs the device found
- The session writer had its own copy of the attachment JSON and forgot
  `for_user`: the file survived the file, but came back as an attachment --
  no card, and put back in front of the model. It now uses the one writer
  (cst_attachments_to_json()), with the round trip covered by a test.
- engineToolImages() looked the card up with findTool(), which only finds a
  card that is still running -- and the engine reports the result *before*
  the files, so the card was already complete and the files were dropped.
  A live turn therefore showed no card while a replay of the same session
  did. It now matches on the call id.
- A tap that could not work said nothing: ImageStrip.qml's errorText was
  never drawn. The card now carries the reason ("no app on the phone opens
  report.md").

Also
- The saved provider key reaches the engine on every start: AgentCore::open()
  was handed an empty key and the engine never resolves credentials itself
  (only the CLI did), so every start asked for it again. It resolves the same
  order main.c uses (saved key, provider environment, CST_API_KEY) and wipes
  the copy it made; --selftest checks it against a scratch credentials file.
- Android: notification_send reads the Java exception text into its result,
  and keeps failing on the NX789J -- the build() returns null and Qt clears
  the exception before C++ can read it, so the call has to be wrapped in Java
  next (docs/android.md says so).
- --uitest grew the delivered-file case (card, Open, Share, a forged name
  refused), the live order of result-then-files, and the startup row count.

Version 0.9.2.

src/tools/fs_tools.c

@@ -291,6 +291,56 @@ int cst_tool_view_image(const cst_tool_ctx_t *ctx, const ctl_tool_call_t *c,
   return rc;
 }
 
+/* --- deliver_file -------------------------------------------------------- */
+
+/* The file IS the answer: copy it into the session store and hand it to the
+ * person as a card the front end can open or share. It is marked for_user, so
+ * the model never sees the copy again -- the result line has to say what was
+ * delivered. */
+int cst_tool_deliver_file(const cst_tool_ctx_t *ctx, const ctl_tool_call_t *c,
+                          cst_tool_result_t *r) {
+  const char *path = tool_arg(c, "path");
+  if (!path || !path[0])
+    return tool_set_result(r, "error: missing path", 1);
+  const char *session = ctx->agent ? cst_agent_session_path(ctx->agent) : NULL;
+  if (!session || !session[0])
+    return tool_set_result(r, "error: no session to store the file in", 1);
+  char full[OS_PATH_MAX];
+  if (resolve(ctx, path, full, sizeof(full)) != 0)
+    return tool_set_resultf(r, 1,
+                            "error: path outside workspace or missing parent: "
+                            "%.200s",
+                            path);
+  if (is_dir(full))
+    return tool_set_resultf(r, 1, "error: %.200s is a directory", path);
+
+  char err[256];
+  cst_attachment_t *a =
+      cst_attach_import_artifact(session, full, tool_arg(c, "title"), err,
+                                 sizeof(err));
+  if (!a)
+    return tool_set_resultf(r, 1, "error: cannot deliver %.200s: %.200s", path,
+                            err);
+  char human[32];
+  if (a->bytes >= 1024 * 1024)
+    snprintf(human, sizeof(human), "%.1f MB",
+             (double)a->bytes / (1024.0 * 1024.0));
+  else if (a->bytes >= 1024)
+    snprintf(human, sizeof(human), "%.0f KB", (double)a->bytes / 1024.0);
+  else
+    snprintf(human, sizeof(human), "%lld B", a->bytes);
+  cst_buf_t out = {0};
+  cst_buf_printf(&out, "delivered %.120s (%s, %s)\n", a->name, a->mime, human);
+  cst_buf_puts(&out,
+               "(the user sees it as a file card they can open or share; "
+               "you will not be shown its bytes again)\n");
+  /* After tool_take_buf: it clears the result, and view_image does the same. */
+  int rc = tool_take_buf(r, &out, 0);
+  r->attach = a; /* one entry; freed as a block by cst_tool_result_free */
+  r->nattach = 1;
+  return rc;
+}
+
 /* --- edit_file ----------------------------------------------------------- */
 
 static size_t count_occ(const char *hay, size_t hn, const char *needle,

src/tools/registry.c

@@ -67,6 +67,17 @@ static const cst_tool_def_t TOOLS[] = {
      {{"path", S, 1, "image file path relative to the workspace"},
       {"detail", S, 0, "low or high (provider hint; default auto)"}},
      CST_TP_READ, 0, cst_tool_view_image, NULL, NO_DYN, NO_META},
+    {"deliver_file",
+     "Hand a finished file to the person: it is copied into the session store "
+     "and shown in the transcript as a card they can open or share. Use it "
+     "whenever the answer is a file -- a report, an export, a patch, a "
+     "picture, a build. You are not shown the copy again, so say what it "
+     "contains in your answer.",
+     "Deliver a workspace file to the user (a file card in the transcript, "
+     "openable and shareable from there).",
+     {{"path", S, 1, "file path relative to the workspace"},
+      {"title", S, 0, "display name (default: the file name)"}},
+     CST_TP_READ, 0, cst_tool_deliver_file, NULL, NO_DYN, NO_META},
     {"run_shell",
      "Run a non-interactive shell command in the workspace root: builds, "
      "tests, git, scripts, computations, data processing, other CLIs "

src/tools/tools_internal.h

@@ -57,6 +57,8 @@ int cst_tool_web_fetch(const cst_tool_ctx_t *ctx, const ctl_tool_call_t *c,
                        cst_tool_result_t *r);
 int cst_tool_view_image(const cst_tool_ctx_t *ctx, const ctl_tool_call_t *c,
                         cst_tool_result_t *r);
+int cst_tool_deliver_file(const cst_tool_ctx_t *ctx, const ctl_tool_call_t *c,
+                          cst_tool_result_t *r);
 
 int cst_preview_write_file(const cst_tool_ctx_t *ctx,
                            const ctl_tool_call_t *c, char **out);