Activity tab: session controls + live usage block (T-415)

The Activity tab gains the power-panel's session strip and a usage block:

- SESSION controls (clear / compact / fork / resume + refresh-usage)
  publish their slash command on builtin.claude/command — the same path
  as typing it, so /clear semantics (and any future confirm behavior)
  live in exactly one place.
- The usage block revisits T-158's "blocked on upstream": probed against
  claude 2.1.175, a forwarded /usage IS answered headless, free
  (num_turns 0), as parseable text. parseUsageText() extracts session /
  week / week-Sonnet percentages (timezone parentheticals stripped); the
  sidebar watches the primary session's synthetic output for
  usage-shaped responses and renders them as a USAGE section. Refresh is
  user-initiated (the control sends /usage) — no polling, no background
  calls (D-64).
- The runtime row gains the session's effort level (T-412's status
  field).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -18,6 +18,11 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Added
 
+- **Session controls and live usage in the Claude sidebar Activity tab.** A
+  SESSION strip offers clear/compact/fork/resume buttons (same code path as the
+  typed commands), and a refresh control fetches `/usage` — plan usage renders
+  as a USAGE block (session and weekly percentages). The runtime row now also
+  shows the session's effort level. (T-415)
 - **The Claude sidebar Config tab is a live control panel.** Model, effort, and
   permission mode are popover controls showing the running session's values;
   picking an option drives the session through the same path as the typed slash