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.

app/src/core/ToolProvider.h

@@ -54,6 +54,20 @@ int platformRegisterMac(AgentCore *core);
 int platformRegisterWindows(AgentCore *core);
 int platformRegisterAndroid(AgentCore *core);
 
+/* Hand a file of the session's store to the system: a phone opens it with
+ * ACTION_VIEW through the app's FileProvider, a desktop gives it to whatever
+ * the desktop uses. `title` is the name a person knows the file by (the
+ * stored copy is named by its id, which means nothing outside), used for the
+ * status line. Both return "" on success and a reason to show the user
+ * otherwise; *status (optional) gets the line to put in the header.
+ * CST_APP_NO_OPEN=1 makes both do nothing (--uitest drives the window without
+ * opening anything for real). */
+QString openLocalFile(const QString &path, const QString &title, QString *status);
+/* The same file through the share sheet where there is one (Android); a
+ * desktop has no share sheet, so the path goes to the clipboard. */
+QString shareLocalFile(const QString &path, const QString &title,
+                       QString *status);
+
 }  // namespace app
 
 #endif /* CST_APP_TOOLPROVIDER_H */