feat(sidebar): cross-device remote status rings

[danshapiro]

What

In the left panel ("Coding Agents" sidebar), a session that is open or busy on a different device now shows a ring around its provider icon on this device:

• Blue ring — the session is busy on another device (any of its remotely-open panes is busy, per the same resolvePaneActivity logic that drives the local blue icon).
• Green ring — the session is open on another device but idle there.
• No ring when the session is open on this device (any window — including the canonical fresh-agent restore-gap identity and sameDeviceOpen records). Blue wins over green, mirroring local precedence.

Non-color carriers included: data-remote-status attribute, tooltip line, and sr-only text (a11y / automation).

How

• Producer (per device, client): the existing 5s tabs.sync.push snapshot now stamps each pane payload with sessionKeys: string[] (complete identity contract: explicit sessionRef, Claude resumeSessionId, Codex durability, live fresh-agent identity, terminal: fallback) and busySessionKeys: string[] (the effective busy identity only — alias identities stay green, matching the producing device).
• Transport: zero server changes. Pane payloads are opaque records; both the Node and Rust servers validate → store → return the new fields verbatim (validated pre-implementation; pinned by tests below).
• Freshness: new 30s periodic tabs.sync.query (previously only on WS ready/reconnect) with trigger precedence — event triggers (ready/reconnect/retention) always supersede immediately; the interval skips while a query is outstanding with bounded 90s stale replacement.
• Consumer: selectRemoteSessionActivity / selectSameDeviceSessionKeys selectors + Sidebar gate (collectSessionRefsFromTabs ∪ collectPaneIdentityActivity ∪ collectBusySessionKeys, plus same-device keys). Comparator discipline maintained (areSidebarItemPropsEqual).

Tests

• Unit/integration: focused suites per task (pane-activity, tab-registry-snapshot, tabRegistrySync, tabsRegistrySelectors, Sidebar) — all green; full npm run check green at the reviewed HEAD (client/server/electron + typecheck).
• E2E (new spec sidebar-remote-status-rings-rust.spec.ts, real Rust server): baseline→busy→open→absent across reloads; no-reload liveness through the 30s periodic query; suppression for locally-open and crafted same-device records; revision-bumped, ack-awaited raw-WS pushes from a second device.
• Node wire pin: ws-tabs-registry.test.ts asserts byte-identical sessionKeys/busySessionKeys round-trip.

Notes

• Produced via the-usual workflow (plan → load-bearing validation → 3 plan-review rounds → TDD execution with per-task + whole-branch review → 2-round independent delta review, final PASSED).
• The plan-review loop ended at its 3-round cap without a final PASSED verdict; all round-3 findings were fixed in the final plan revision (d484f4275) and the delta passed independent review at 8043ce20e.