docs(mcp): encode rule-scope model in rulebook tool descriptions

Make the always-on / subscribed / project-rule distinction explicit at the
authoring surface so it can't silently regress (for this operator or other
users). Previously the tools said only 'cross-project rulebook rule' and a
bare 'subscribe a project' — nothing steered project-specific detail away
from shared rulebooks, which is how a Scribe-pinned rule ends up binding
every family project.

Principle encoded in 5 places: a rule's home is chosen by WHO it should bind,
and both rulebook tiers are SHARED so their rules stay general — they differ
in reach (all projects vs opt-in by theme), not generality. Project-specific
detail goes in create_project_rule.

- server.py MCP instructions: add the 3-tier authoring principle
- create_rule / create_rulebook / create_project_rule / subscribe_* docstrings
- using-scribe SKILL.md: a 'Where a new rule goes' note for the pull path

Refs #755

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugin/skills/using-scribe/SKILL.md

@@ -48,6 +48,25 @@ explicit pull. Rules loaded this way are **binding** for the session.
 5. **Keep state honest.** Set a task `in_progress` when you start it, `done` the
    moment it's complete; log progress as you go.
 
+## Where a new rule goes
+
+When codifying a rule, pick its home by **who it should bind** — and keep
+shared homes general:
+
+- **Always-on rulebook** (`create_rule` in an `always_on` rulebook) — universal
+  norms that bind *every* project. Cross-project standards only.
+- **Subscribed rulebook** (`create_rule` + `subscribe_project_to_rulebook`) — a
+  reusable, *themed* module of general rules that binds only projects that opt
+  in (e.g. a design system → visual apps). Themed, but still project-agnostic.
+- **Project rule** (`create_project_rule`) — anything specific to one project
+  (its files, paths, quirks).
+
+Both rulebook tiers are shared, so their rules stay general; they differ in
+**reach** (all vs opt-in), not generality. Names one project's specifics →
+project rule; a standard a category shares → subscribed rulebook; a universal
+norm → always-on rulebook. Never put project-specific detail in a shared
+rulebook — it leaks to every other project that gets it.
+
 ## Other Scribe process-skills
 
 This plugin also ships focused process-skills — brainstorming, systematic