feat(agentos): bind FleetCockpit ActivityStream to the fleetActivity feed + honest sample/live/stale states

Sub of #14606 (FM cockpit ActivityStream live-binding) — the app-side consumption of the merged #14863 fleetActivity capability.

The shipped shell renders the ActivityStream with a hardcoded ● streaming header over fixture events — a liveness misstatement. This slice binds the stream to the real feed and states its source honestly.

Scope

• FleetCockpit polls the read-observe fleetActivity verb on globalThis.AgentOS.fleet.registryBridge (the capability merged in #14863) on construct, fail-closed.
• Routes the feed's honest capability state to the stream: wired + events → live (feed is newest-first; the stream renders chronological, so reversed); not-wired / absent bridge → the representative sample stays, honestly labelled; degraded → the stale banner. Never presents the sample as live.
• ActivityStream: new sample header state (sample · live feed pending); eventText now reads payload.subject (the live A2A/PR/lane adapter's text field) in addition to the fixture's payload.text.

Acceptance Criteria

• eventText renders payload.subject meaningfully (not the agentId · type fallback).
• adapterState: 'sample' renders sample · live feed pending (never streaming); stale renders the degrade banner; both show rows, never blank.
• FleetCockpit.loadActivity is fail-closed: wired→live, not-wired/no-bridge→sample, degraded→stale, error→sample (no boot break).
• Unit-tested (extends activityStream.spec.mjs).

Out of scope (remaining #14606 + infra)

• The orchestrator-side real activitySource wiring (devFleetServer ↔ memory-core mailbox access) — a separate infra lane; until it lands the feed is honest sample.
• The NL-verifiable live e2e mount (on #14859's hardened fixture).

Filed by Vega (@neo-opus-vega).