fix(tui): rename sidebar Tasks panel to Activity (#4147)

[Hmbown]

Fixes #4147

Summary

The sidebar panel titled "Tasks" actually renders live tools and background jobs, not durable task state, causing a confusing label/content mismatch. This renames the panel's user-facing label to "Activity" so the title matches what it shows and no longer collides with durable tasks. This is a surgical copy/label change — one rendered title string plus the module doc-comment — with no renaming of Rust modules, structs, or functions (guard #4172).

"Activity" was chosen over the "Running"/"Live" fallbacks because it does not collide with any existing sidebar section title (Pinned / To-do / Agents / Session / Hotbar).

Acceptance criteria

• AC1 — No user-facing collision with durable tasks: the sidebar title is now "Activity"; the literal user-facing "Tasks" title is gone (internal task_panel/SidebarFocus::Tasks identifiers are unchanged per guard v0.9.0 guard: preserve verified active modules and DEEPSEEK env names #4172).
• AC2 — Label matches contents (live activity): the panel renders live tools / background jobs, and "Activity" describes exactly that.
• AC3 — Distinct from the WorkflowPanel above the input (v0.9.0 2.4: add WorkflowPanel unified activity surface #4121): no concepts merged; the Activity panel and WorkflowPanel remain separate (no WorkflowPanel references in sidebar.rs).

Verification

• cargo fmt --all -- --check — clean
• cargo clippy --workspace --all-features --locked -- -D warnings <CI allow-list> — clean (exit 0)
• cargo test --workspace --all-features --locked — passes except the pre-existing, environment-dependent startup-skill-cache test skill_hotbar_action_activates_skill_through_dollar_alias, which fails identically on the clean base commit without this change (unrelated to a sidebar label edit)
• cargo build --release — succeeds

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Claude's Review — PR #4205

• Gather context (diff, changed file)
• Review correctness / provider-safety / reuse / tests / security per repo review priorities
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Summary

This is a small, well-scoped, low-risk copy change. I verified the diff (crates/tui/src/tui/sidebar.rs, +6/-2) directly rather than relying only on the PR description.

What it does:

• Renames the sidebar panel's user-facing title from "Tasks" to "Activity" (sidebar.rs:1256, in render_sidebar_tasks).
• Updates the module doc-comment header to match (Pinned / Tasks / Agents / Context → Pinned / Activity / Agents / Context).
• Adds an explanatory comment tying the rename to v0.9.0 5.9: rename sidebar Tasks panel to match live activity semantics #4147 and noting the internal identifiers (task_panel, SidebarFocus::Tasks) intentionally stay unchanged per guard v0.9.0 guard: preserve verified active modules and DEEPSEEK env names #4172.
• No Rust identifiers, structs, or module names touched.

Review findings (by priority)

1. Correctness — No bugs. This is a literal string swap; no logic, control flow, or match arms are touched. No panics/unwraps introduced.
2. Provider/model/route safety (EPIC v0.8.65 EPIC: Separate provider facts, model facts, offerings, and route resolution #2608) — Not applicable; no provider/model/routing code touched.
3. Reuse/simplification — N/A for a diff this size; no duplication or dead code introduced.
4. Tests — I searched the tui crate for any test or snapshot asserting the literal "Tasks" title string and found none, so nothing is left broken by the rename. No new test was added, but none is really warranted for a static label string — a snapshot/golden test would be the only thing that could regress here, and there isn't one to update.
5. Security — N/A, no security-relevant surface.

Verified independently:

• Grepped crates/tui/src for "Tasks" / "Activity" string literals — the only remaining "Activity" usage is the one added here, and no other sidebar section title collides with it ("To-do", "Agents", "Session", plus Pinned/Hotbar elsewhere) — confirms AC1/AC3.
• Confirmed no other "Tasks" literal remained anywhere in the tui crate that would now be stale/inconsistent.
• Read the surrounding function (render_sidebar_tasks) — the change is exactly the title string plus the doc-comment, as described.

Note: I wasn't able to independently run cargo fmt --all -- --check / cargo clippy / cargo test in this sandbox (Bash tool required approval that wasn't available for cargo invocations). The PR description states these were run and pass cleanly with one pre-existing, unrelated test failure (skill_hotbar_action_activates_skill_through_dollar_alias) reproducible on the base commit — I have no reason to doubt this given the diff's size and nature, but flagging that I couldn't verify it myself. If you'd like me to actually run the build/test/lint suite, update the allowed tools to permit cargo commands.

Verdict: Looks good — surgical, matches its stated acceptance criteria, no correctness or safety concerns found. No changes requested.