fix(vscode): clear completed session indicator when its tab is focused - #13703

Fixes #13702

Context

Follow-up to #13504, which added session-activity icons to tabs. A session's completed (✓) indicator persists on its tab even after you open that session to review it. The check only clears on a new turn, revert, resume, submit, or delete — so when a background session finishes, you see the ✓ from another tab, click in to review, and the ✓ stays. That makes it hard to tell, across many tabs, which completed sessions you've already looked at.

Implementation

The activity state for every surface (sidebar tabs, native editor tabs, Agent Manager tabs, worktree cards) is derived from one shared map computed from closeMap (outcomes), statuses, blocked, and submitting — notably not from which tab is focused. The completed outcome is only dropped by clearClose(id), whose callers are new turn / revert / resume / submit / delete.

This adds one more trigger: when currentSessionID changes to a session that is already in the done state, clear its close outcome. It's keyed on the focus transition via on(currentSessionID, ...), deliberately not a continuous effect — so a session that completes while it's already the focused tab still shows its check (it only clears once you navigate away and back). Because all surfaces read the same derived map, clearing in this one place updates every surface.

Scope is intentionally narrow: only done is acknowledged on focus. waiting (needs input/permission) and error are never cleared by focus, since those are unresolved states that must persist until the user actually acts.

Screenshots / Video

How to Test

Manual/local verification

• Ran the affected regression, extended to cover this behavior: bun test tests/unit/session-provider-activity.test.ts — passes. Added assertions cover (a) a session that completes while another tab is focused shows done, then clears when you switch to it, and (b) the guardrail: switching to a tab in the waiting state does not clear it.
• bun run check-types:webview — clean.
• eslint on the two changed files — clean.
• Built the extension and verified in an isolated VS Code dev host: with two sessions, let one finish while focused on the other; the ✓ shows on the background tab and clears on clicking into it; a ⚠ needs-input tab does not clear on click.

Reviewer test steps

1. Open two Kilo sessions (sidebar tabs or Agent Manager).
2. Start a turn in session A, switch to session B.
3. When A completes, confirm B's tab is unaffected and A's tab shows the green ✓.
4. Click A. Confirm the ✓ clears now that A is focused.
5. Trigger a permission/question in a third session while on another tab, then click into it without answering — confirm the amber ⚠ persists (only done clears on focus).