feat(workspace): add registry-global archivedSessionIds set

The workspace domain global singleton gains archivedSessionIds (schema
default [], version unchanged): a display-layer archive set layered over
workspace accounting. archiveSession() rides the registry operation
chain, validates the session against live/persisted headers, and is
idempotent; archived sessions keep their sessionIds slot so a future
unarchive restores position.

packages/workspace/workspace/README.md

@@ -13,6 +13,7 @@ The entity/storage rationale lives in the [domain Agent Note](../../../.agents/n
 - `ctx.workspace.delete(id)` — removes only the Workspace registration, its durable order entry, and its session account. Unknown ids return `false`; a removed record returns `true`. The directory, user files, live Sessions, and persisted session logs are never touched, so those Sessions become Ungrouped. A table-write failure restores the prior order and published entity.
 - `Workspace.attachSession(id)` — validates a live or persisted session header cwd against the workspace path and prepends a new id. Unknown sessions, absent/unresolvable/non-directory cwd values, and mismatches reject without writing. `detachSession` removes only the candidate index entry.
 - `ctx.workspace.touchSession(id)` — moves only that validated, accounted session to the front. Ungrouped or filtered sessions are no-ops, and workspace order never changes.
+- `ctx.workspace.archiveSession(id)` / `archivedSessionIds` — the registry-global archive set, layered over workspace accounting: an archived session disappears from grouping surfaces but keeps its session log and its `sessionIds` slot, so a future unarchive restores its position. Archiving accepts any live or persisted session (accounted or Ungrouped), resolves without writing for an already archived id, and rejects an unknown id. State written before the field existed parses with an empty set.
 - `Workspace.sessionIds` — synchronous id-plus-canonical-cwd membership projection in durable candidate order. Missing headers, invalid cwd values, and mismatches are filtered; the next workspace mutation prunes them. A medium indexing one session under two workspaces, claiming one path from two records, or diverging from durable workspace order rejects at startup.
 - `Workspace.status()` — uncached directory check, `'ok' | 'missing-dir'`; a missing directory never mutates the record.