feat(6855): the plugin asks before a session writes to a project other than its own

plugin/hooks/grimoire_project_fence.sh runs as PreToolUse on editor writes,
Bash and Grimoire's tools. A session belongs to the project its starting
directory resolves to (CLAUDE_PROJECT_DIR: a .grimoire marker, else the git
remote's binding). A session that starts outside any project belongs to the
first one it enters, which PostToolUse on enter_project records.

These acts stop on permissionDecision "ask", with the server's reason naming
the other project:
- an edit in another project's repo or a worktree of it;
- a git commit, push, merge, rebase, pull, cherry-pick, revert, tag, am, reset
  or worktree add there, found through git -C, a cd before it, or the shell's
  current directory;
- a Grimoire write that lands in another project.

A live probe showed "ask" holds in bypassPermissions. Filing an issue and
every read go ahead. An edit or git command in the session's own repo, and any
tool named like a read, never calls the server. Grimoire writes send only
their short arguments, so the URL stays small.

When the instance doesn't answer, the act goes ahead and the hook says once
per outage that it couldn't check. check_plugin.py now holds one outage line
per hook and has a smoke event for the fence.

GET /api/plugin/write-scope takes fallback_project_id for the entered project.

tests/test_project_fence.py drives the hook against real git repos and a
stand-in server. tests/test_write_scope.py holds the hook's
FENCE_READ_PREFIXES equal to services/write_scope.READ_PREFIXES. The plugin
README and PACKAGING describe the fence, and the plugin is minted to
2026.11.05.0245.

Milestone 583, step 3.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/grimoire/routes/plugin.py

@@ -637,7 +637,9 @@ async def write_scope():
     before a file edit, a git command or a Grimoire write.
 
     A session belongs to the project its starting directory resolves to
-    (`repo` / `project_id`, as every plugin endpoint reads them). The act is
+    (`repo` / `project_id`, as every plugin endpoint reads them), or when
+    that resolves to none, to `fallback_project_id`: the first project the
+    session entered. The act is
     either a directory (`target_repo` / `target_project_id`) or an MCP call
     (`tool`, with `input` as a JSON object of the call's short arguments: the
     ids, kind and status that say where it writes). `act` words the act for the
@@ -648,6 +650,13 @@ async def write_scope():
     file edits and git commands.
     """
     session_project, _repo, _unbound = await _project_scope()
+    if not session_project:
+        # A session that started outside any project belongs to the first one
+        # it entered; the hook remembers it.
+        try:
+            session_project = int(request.args.get("fallback_project_id") or 0)
+        except ValueError:
+            session_project = 0
     if session_project and not await can_read_project(g.user.id, session_project):
         session_project = 0
     tool = (request.args.get("tool") or "").strip()