feat(workspace): direct backend (default), watch tool, write { append }

changelog.d/workspace-direct-backend.added.md

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+- Workspace mounts take a `backend` (recipe: `modules.workspace.mounts[].backend`).
+  With `'direct'` — now the default — the same tools (`read`, `write`, `edit`,
+  `delete`, `ls`, `glob`, `grep`, `read_image`) operate on the mounted
+  directory itself: no Chronicle tree state, so there is no second copy that
+  can go stale behind shell-side edits, and nothing to materialize or sync.
+  `materialize` and `sync` report a direct mount under `skipped` instead of
+  touching it; `status` and a path-less `ls` show each mount's backend. Direct
+  writes are atomic (temp file + rename), `edit` refuses a file another writer
+  changed since it was read, containment and symlink rules match the
+  chronicle backend's disk reads, and recursive listings report `truncated`
+  when they stop at the file cap rather than going quiet. `backend:
+  'chronicle'` keeps the previous behaviour (branch-scoped, undoable edits
+  that reach disk on materialize). A mount switched back from direct to
+  chronicle syncs disk into the store before its first materialize, so the
+  stale tree never overwrites disk.
+- `write` takes `append: true` on both backends, adding to the end of the file
+  (creating it if missing) instead of replacing it. On a direct mount the
+  append is a single `O_APPEND` write, safe to interleave with a shell `>>`.
+- New workspace tool `watch`: the agent registers a file or directory (or a
+  whole mount) and is woken, with a context line naming the path and op,
+  whenever something other than its own tools creates, modifies or deletes
+  under it. Watches persist in module state across restarts. On a direct mount
+  the watcher starts on demand; a chronicle mount needs `watch: 'always'`.

changelog.d/workspace-direct-default.breaking.md

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+- **Recipe authors and hosts:** workspace mounts now default to
+  `backend: 'direct'`. A mount that omits `backend` reads and writes its
+  directory directly: its Chronicle tree state is no longer registered or
+  consulted, `materialize`/`sync` skip it, and its edits are not branch-scoped
+  or undoable. Set `backend: 'chronicle'` on any mount that needs the tree
+  (a config mount the host rewinds with `materializeMount`, a mount whose
+  files must follow conversation branches). Hosts that construct
+  `MountConfig`s programmatically must pass `backend` through from the recipe
+  and pin their own tree-dependent mounts to `'chronicle'`. Existing stores
+  keep their tree states untouched; upgrading a mount to direct abandons the
+  tree in place (disk is what the resident already saw via shell), and a
+  later switch back syncs disk into the store before the first materialize.