feat(composer): terse tool-call headline for the activity pill

- Rewrite the tool branch of getActivityHeadline (agentSessionTranscriptPresentation.ts)
  to use the descriptor's terse action tier — verb + compact object, e.g.
  "Read foo.ts" — instead of "label · full preview" ("Read file ·
  src/agents/ui/foo.ts"), so the composer pill's 200px cap shows the
  informative part of the action instead of ellipsizing a long preview
- Basename path-like objects only (file-read / file-edit / skill-read);
  shell commands stay whole since they legitimately contain "/", and file
  edits reuse the already-basenamed fileEditSummary.filename
- Fall back to the previous "label · preview" format when a descriptor
  carries no action (older descriptors)
- Export the existing private basename() from agentSessionFileEditDiff.ts
  rather than adding a third copy
- Document the terse format on deriveActivityPillLabel (composerLiveActivity.ts);
  no logic change — the pill picks it up via getActivityHeadline, whose only
  production consumer is the pill
- Tests: update presentation expectations ("Send Message · abc" → "Sent abc"),
  add cases for file-read/file-edit basenaming, shell commands kept whole,
  and the no-action fallback; add a pill-level terse-headline case to
  composerLiveActivity.test.mjs

Co-authored-by: Taylor Ho <taylorkmho@gmail.com>
Signed-off-by: Taylor Ho <taylorkmho@gmail.com>

desktop/src/features/agents/ui/agentSessionFileEditDiff.ts

@@ -197,7 +197,8 @@ function getDiffStats(
   return null;
 }
 
-function basename(path: string) {
+/** Final path segment (cross-platform); returns the input when empty. */
+export function basename(path: string) {
   const parts = path.replace(/\\/g, "/").split("/");
   return parts[parts.length - 1] || path;
 }