Add ability to unarchive / restore an archived project session

## Scenario

I accidentally archived a long-running project session that I'd been using as an orchestrator (it had spawned many child sessions via cross-session messages, accumulated significant context across many turns, and had a checkpoint summarizing its state).

When I realized the mistake and tried to resume it, I couldn't find a way to bring the session back. As far as I can tell:

- Archived sessions aren't visible in the normal project session list
- There's no "unarchive" or "restore" action exposed in the UI or via tooling
- The session's state still exists (history, checkpoints, branch, worktree all intact) — it just isn't reachable

The only workaround I found was to create a **new** session and seed its kickoff prompt with a hand-assembled context dump from the previous session's last checkpoint. That works, but:

- It's a clone, not a resume — the new session has a different ID, so any cross-session references from child sessions still point at the dead parent
- It requires me (or an agent) to manually reconstruct context, which is error-prone and loses fidelity
- Any in-flight automations tied to the original session ID are orphaned

## Request

Add a first-class way to **unarchive / restore an archived project session**, ideally:

1. An "Archived" view or filter in the project session list
2. A restore action that brings the session back with the same ID, branch, worktree, and history intact
3. (Optional) Equivalent tooling exposure so agents can recover from accidental archives without the kickoff-prompt workaround

## Why this matters

Archive is a one-click action and easy to hit by mistake. For short-lived sessions the loss is minor, but for long-running orchestrator/parent sessions — the exact ones most valuable to preserve — an accidental archive is currently destructive in practice even though the data is still there.

Start by tracing how archived sessions are excluded from the project session list and how the archive action persists session state. Define the restore path around the existing history, checkpoints, branch, worktree, and session ID, then consider equivalent tooling exposure. Done means an archived session can be found and restored without cloning it, with its identity and state intact.

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