feat(desktop): stable Slack-style status line in the composer activity bar

[TolgaCinisli]

Summary

The inline bot activity bar rotated through the last 5 distinct transcript
headlines every 2.2s. During a long agent turn that reads as flicker — three or
four unrelated strings looping — and there is no way to tell how far along the
turn is.

Replace the rotation with one line that updates in place:

Assistant · Bash: npm test · 2m 10s · 12 tools · ctx 118K/1M

• Current activity — the running tool with a clamped preview, or the turn
  phase (Thinking / Planning / Responding) when no tool is in flight.
• Elapsed — ticks once a second, anchored to the agent working signal.
• Tool count — per turn, resets when a new turn starts; suppressed rows
  excluded.
• Context — the latest usage reading, session-level so it survives turns.

Shell rows prefer the human-authored description argument over the raw
command text. The bar is a status line, not a terminal — sed -n '420,432p' src/… reads as noise there, while "Read the store cache" is the thing a person
wants. The session panel still shows the full command.

The projection is a pure function in a new botActivityStatus.ts; only the
elapsed segment needs the component's clock. Popover and session panel are
untouched.

Related issue

None found — searched open issues and PRs for "activity bar" / "status line" /
"rotating".

Testing

• New botActivityStatus.test.mjs — 11 cases covering running-tool preference,
  phase fallback, turn/channel scoping, suppressed rows, usage parsing,
  basename shortening, the idle shape, segment formatting, and the
  description-over-command preference.
• pnpm -C desktop test — 4052 passing
• pnpm -C desktop typecheck — clean
• biome check on the touched files — clean
• Manual: ran multi-minute agent turns in a channel; the line updates in place
  with no flicker and the counters reset cleanly on the next turn.