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

tests/test_services_plugin_context.py

@@ -215,6 +215,31 @@ async def test_build_session_context_unbound_repo_emits_bind_hint():
     assert "## Active project" not in ctx
 
 
+@pytest.mark.asyncio
+async def test_a_project_id_that_does_not_resolve_is_reported_not_dropped():
+    """A `.scribe` marker names a project directly (#4085), so for the first
+    time a caller arrives holding a pointer it BELIEVES in. If the id is for
+    another instance, or names a deleted project, the session has to be told —
+    it cannot infer it from an absence.
+
+    This used to render nothing whatsoever: the branch hung off `if project_id`
+    as an `elif`, so an id that was sent and failed took the outer arm, found
+    no project, and fell out of the block having said nothing at all.
+    """
+    from scribe.services.plugin_context import build_session_context
+    with patch("scribe.services.plugin_context.projects_svc.get_project",
+               AsyncMock(return_value=None)):
+        out = await build_session_context(user_id=7, project_id=41)
+
+    ctx = out["context"]
+    assert out["project"] is None
+    assert "## Project 41 could not be loaded" in ctx
+    assert "list_projects" in ctx
+    # Not mistaken for the repo case, which has a different remedy.
+    assert "## Repository not yet bound" not in ctx
+    assert "No Scribe project is bound" not in ctx
+
+
 @pytest.mark.asyncio
 async def test_build_process_manifest_renders_stub_specs():
     items = [