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.

docs/android.md

@@ -158,10 +158,27 @@ have, so the script rewrites the shebang of the build-tools wrappers.
 * The APK builds, installs and runs: the Qt front end links the engine
   in-process (no port), `main.cpp` puts the workspace, config and tmp under
   `/data/user/0/xyz.whcself.cstagent/files/`, and a turn does a real HTTPS
-  request from inside the ART process (with no API key the provider answers
-  401 -- the engine, curl and TLS parts of the round trip are what that
-  shows). Rotation keeps the session; the sidebar appears in landscape.
-* Still to come: the Android platform tools behind `app:android`
-  (`notification_send`, clipboard, share sheet, SAF file picker), the back
-  gesture, and the icon glyphs -- `ThemeIcon` finds no icon files on the phone
-  and draws empty boxes where the desktop shows Breeze icons.
+  request from inside the ART process. The saved provider key is resolved into
+  the engine's config on every start (`AgentCore::open()`: saved key, the
+  provider's environment variable, `CST_API_KEY`), so the phone answers without
+  being asked for the key again. Rotation keeps the session; the sidebar
+  appears in landscape.
+* Platform tools behind `app:android` (`app/src/platform/android/AndroidTools.cpp`):
+  `clipboard_get` / `clipboard_set` under the engine's own tool names and
+  `share` (`ACTION_SEND` + chooser) both work on the device. The file
+  card's **Open** builds a `content://` URI through the app's FileProvider
+  (`<package>.qtprovider`, Qt's `@xml/qtprovider_paths`), checks
+  `resolveActivity()` before starting `ACTION_VIEW` and grants the reader
+  permission; **Share** reuses the share sheet.
+  `notification_send` still fails on the NX789J: the channel and the small
+  icon are found, `Notification.Builder.build()` returns null, and Qt clears
+  the Java exception before C++ can read it ("no exception detail"). Catching
+  it needs the call to be wrapped in Java (a small `app/android/src/...`
+  helper class), which is the next step there.
+* A session's transcript replays a delivered file (`deliver_file`) as a card
+  with Open and Share -- the copy lives next to the session, marked `for_user`
+  so no request ever contains it (see `app/README.md`, "Files the agent
+  delivers").
+* Still to come: the SAF picker for attaching a picture from another app, the
+  back gesture, and the icon glyphs -- `ThemeIcon` finds no icon files on the
+  phone and draws empty boxes where the desktop shows Breeze icons.