Problem

When a workspace's anchor path becomes invalid (worktree deleted, directory moved/deleted), the workspace ID remains in the agent's attached_workspaces list forever. Only detach_workspace removes entries; all other worktree cleanup paths (cleanup_task_owned_worktree_in_detail, discard_managed_worktree, exit_worktree) delete the physical worktree but never clean up the agent state.

This causes stale workspace entries to accumulate in the UI (workspace IDs with no path shown, not browsable).

Closes #2030.

Solution

Add prune_stale_attached_workspaces() to RuntimeHandle that lazily removes stale workspace IDs from attached_workspaces. An entry is stale when:

• Its workspace ID is absent from the workspace registry, or
• The registry entry's anchor path no longer exists on disk

The prune runs from the agent_state HTTP handler before building the summary, so stale entries are cleaned on every state API read — the primary read path for the UI.

Design choice: runtime-layer cleanup vs. projection-only

The issue proposed doing the cleanup inside state_workspace_snapshot (a pure projection function). Instead, the cleanup lives at the runtime layer (RuntimeHandle::prune_stale_attached_workspaces) because:

1. Projection functions should be side-effect-free — state_workspace_snapshot should not mutate agent state.
2. Persistent cleanup — stale IDs are removed from persisted agent state, not just hidden from one API response.
3. Audit trail — each pruned ID emits a workspace_detached audit event with reason: "stale_workspace_anchor".

Safety guarantees

• Active workspace is never pruned, even if its anchor is currently invalid, to avoid silently clearing the execution context.
• AgentHome (agent_home and agent_home:{agent_id}) is never pruned — it is a virtual workspace that may not be in the registry.

Changes

Test plan

• prune_removes_workspace_id_missing_from_registry — stale ID not in registry gets pruned
• prune_removes_workspace_with_deleted_anchor — registry entry with deleted anchor dir gets pruned
• prune_preserves_active_workspace_even_when_stale — active workspace is never pruned
• prune_preserves_valid_workspaces — valid workspaces are preserved
• prune_emits_audit_events_for_removed_workspaces — audit events with correct reason
• cargo fmt --all -- --check passes
• RUSTFLAGS="-D warnings" cargo check --all-targets passes