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

plugin/skills/using-scribe/SKILL.md

@@ -13,10 +13,23 @@ asked for.
 
 ## Do this first (every session)
 
-If the working repo maps to a Scribe project (you're in a known repo, or
-`list_repo_bindings` shows a binding), call `enter_project(id)` — it returns the
-project's goal, the milestones and open tasks worked on most recently, its
-Systems and the titles of its own rules in one shot.
+If the working directory maps to a Scribe project, call `enter_project(id)` —
+it returns the project's goal, the milestones and open tasks worked on most
+recently, its Systems and the titles of its own rules in one shot.
+
+**A directory does not have to be a git repo to have a project.** A repo is
+bound by its remote (`list_repo_bindings` shows the bindings). Anything else —
+a notes folder, a server's config directory, a scratch directory — is bound by
+a `.scribe` file naming the project:
+
+    {"instance": "https://scribe.example.com", "project_id": 2, "project": "Homelab"}
+
+`instance` is what makes the id trustworthy. An id means nothing on its own —
+it is a different project on every Scribe — so a marker that has travelled to
+another instance is ignored rather than followed to the wrong project. A bare
+`2` also works when writing the file by hand. When work plainly belongs to a
+project and the directory names none, offer to write the marker;
+`list_projects` has the id.
 
 Then **ask before you act**: before anything hard to reverse or outward-facing,
 search the rules for what you are about to do. Reflex 2 below is why asking,