feat(rules): enter_project handshake (S4)

New enter_project(project_id) MCP tool composes get_project +
get_applicable_rules + get_project_milestone_summary + recent
open-tasks + recent notes into one round-trip, intended to be called
at session start (or whenever the active project changes) so Claude
has the full project context loaded before it starts mutating.

_INSTRUCTIONS now points Claude at enter_project for project-scoped
work, alongside the existing list_always_on_rules instruction. No
schema change; pure composition over existing services.

Closes the four-slice rules-consolidation plan (Scribe task #508):
S1+S2 (always_on flag + Scribe-first prompt, 658348f), S3 (project-
scoped rules, 43a860c), and now S4.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/fabledassistant/mcp/server.py

@@ -56,6 +56,13 @@ creating a rule, call list_always_on_rules and list_rules(project_id=...) to
 avoid duplicates. Coordinate with the operator on whether a new rule belongs
 in a project, an existing rulebook+topic, or a new rulebook.
 
+When you are working on a specific project, call enter_project(project_id)
+ONCE at session start (or whenever the active project changes). It returns the
+project, its applicable_rules + project_rules + subscribed_rulebooks, milestone
+summary, open tasks, and recent notes — everything you need to know the lay of
+the land before mutating. Don't call get_project + get_applicable_rules + a
+search separately when enter_project already composes them.
+
 Plans are tasks with kind=plan, and Scribe is the canonical home for them.
 When you begin non-trivial work, call start_planning(project_id, title) FIRST —
 before any brainstorming, design, or plan-writing skill runs. start_planning