feat(plugin): a directory says which project it belongs to, git repo or not (#4085)

All six hooks scoped their requests one way: `git remote get-url origin`,
resolved server-side through the repo bindings. That key does not exist
outside a git repo, so a session in a plain directory was unscoped in every
hook at once — no project context, no prior-art scoping, no project rules —
and silently, because a missing remote is indistinguishable from a remote
nobody bound.

A `.scribe` file is the second key, read by the shared scribe_scope_query so a
directory scopes the same way everywhere:

    {"instance": "https://scribe.example.com", "project_id": 2, "project": "…"}

`instance` is why the file is not just a number: an id is a different project
on every Scribe, so a marker that travels — a copied directory, a shared
machine, a repo someone else clones — would otherwise scope the session to the
wrong project without a word. Compared host-only, and a mismatch drops the id:
no project beats the wrong project. A bare integer is accepted too, since it
is what a person writes by hand. The marker beats a git remote — someone put
the file there on purpose — which is also how a directory overrides its
binding.

Two things it found on the way:

  * An explicit project_id that did not resolve rendered NO message at all —
    the branch hung off `if project_id` as an `elif`, so a caller holding a
    pointer it believed in got a context that silently omitted the project it
    had asked for. Now reported.
  * The refusal reason was a global set inside a function every caller reads
    through `$( )`. The assignment died with the subshell, leaving the caller
    to read an unset variable under `set -u` — which aborts the hook and costs
    the whole session's SessionStart context, to fetch a warning about a file.
    It comes back through stdout with the id instead, and a test pins it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

src/scribe/routes/plugin.py

@@ -64,8 +64,11 @@ async def session_context():
             resolves it to the bound project (see services/repo_bindings); an
             unbound repo yields a "bind this repo" hint instead. This is how the
             active project is determined — the plugin never pins a project id.
-        project_id (optional int) — explicit override, mainly for manual/ad-hoc
-            curl testing; takes precedence over `repo` when set.
+        project_id (optional int) — the project named directly, and the only
+            key a caller outside a git repo has (#4085): the plugin's hooks
+            send it when a `.scribe` marker file names a project. Takes
+            precedence over `repo`. Access-checked like any other read — an id
+            this account cannot read loads no project rather than failing.
     """
     project_id, _repo, unbound_repo = await _project_scope()
     result = await plugin_ctx_svc.build_session_context(