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>

plugin/PACKAGING.md

@@ -39,6 +39,7 @@ another one means adding files, not moving or rewriting any.
 | `hooks/grimoire_prior_art.sh` | PreToolUse on editor writes: `GET /api/plugin/prior-art`. |
 | `hooks/grimoire_after_write.sh` | PostToolUse on shell commands: the same check for code written through the shell. |
 | `hooks/grimoire_tool_rules.sh` | PreToolUse on shell commands: `GET /api/plugin/tool-rules`. |
+| `hooks/grimoire_project_fence.sh` | PreToolUse on editor writes, shell commands and Grimoire's tools: `GET /api/plugin/write-scope` for an act aimed at another project, answered `ask` with the reason; PostToolUse on `enter_project` remembers the project a session without one entered. |
 | `hooks/grimoire_moment.sh` | PreToolUse on every tool: the rules mounted on the moments the call reaches, `POST /api/plugin/moment`; skips tools `GET /api/plugin/moment-tools` says reach nothing mounted, and Grimoire's own (their responses carry `moment_rules`). |
 | `hooks/grimoire_reply_check.sh` | Stop: sends the finished reply to `POST /api/plugin/reply-rules` — the rules mounted on the reply moments, and the reply against every rule's trigger, and — when the turn closed a task — the completion-section check; holds once per rule, with the reason the server returns, and never holds the rewrite. |
 | `hooks/grimoire_shape_check.sh` | Stop: sends the definitions the turn wrote (the write hooks' `<sid>.written.ids` ledger) to `GET /api/plugin/shape-check`; blocks once, with the reason the server returns, so the agent judges what it built. |