feat: show last-known Agents when live data is absent refs #237

[aliefe04]

Implements #237: Console persists a last-known Agent cache (versioned UserDefaults blob mirroring PinnedAgentsStore, per-host atomic replace, corrupt/version-mismatch fails closed, 500-row/host cap, no terminal output) written only from authoritative snapshots.\n\n- Cold launch / reconnect gap: cached rows render disabled (plain rows, no NavigationLink/contextMenu) under a non-blocking 'Last known agents' notice; live data always wins; first refresh heals; removing a Host clears its cache.\n- Deep links to ended agents already land on Console via the router's pending-with-grace semantics (pinned by existing AgentNotificationRouterTests: stale-pane expiry + nil-target cases).\n\nCloses #237.\n\nVerified: swiftc -parse per touched file + git diff --check + CHANGELOG refs #237 (PASS). Full tests left to CI (no Xcode SDK on this machine).

[aliefe04]

Final note on the missing CI, with the test I ran: pushing this head produces no run object, and closing + reopening the PR (a reopened pull_request event, evaluated against the full PR diff) also produces none — still check-suites → 0 for the head, and the newest run in the whole repo remains 2026-09-10T17:44:34Z. So this is not the fork-approval gate (those runs exist and show as action_required, e.g. on #312/#313) and not the paths filter; GitHub is creating no workflow runs for this repository at all since that timestamp.

Re-running an older run from the Checks tab would only re-test the previous commit, so that is not a way to verify this head. If you get a chance, could you check the repository's Actions settings (or whether Actions is enabled/limited for the repo) when convenient? The branch itself is rebased onto current main, merges cleanly, and the failures from its last executed run are fixed here.