* fix(workspaces): show provider subagent activity
Provider subagent lifecycle lives outside managed agent snapshots, so workspace aggregation dropped it after the parent turn finished. Count running provider subagents against the delegation root and emit workspace updates when their status changes.
* fix(workspaces): surface cross-workspace subagents
The local/worktree choice was unstored component state and reset on every
remount of the New Workspace screen (e.g. after creating a workspace).
Persist it in the create-form preferences (FormPreferences.isolation).
Also rename the app-layer "backing" to "isolation" to match its UI label.
Isolation is the create-time intent; it stays distinct from workspaceKind,
the git-derived persisted property, and the wire's source.kind.
Multiple workspaces can share one checkout directory. Agent and terminal
status, sidebar tabs, and ownership now attribute strictly by workspace ID,
so a running or blocked agent in one workspace no longer shows up on its
same-directory siblings.
A one-time startup migration backfills workspace IDs onto legacy agents, and
every agent and terminal is created with one thereafter. Directory lookups
remain only for genuinely folder-derived git facts (branch, diff) and the
archive-by-path adapter.
