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

src/fabledassistant/mcp/tools/projects.py

@@ -18,6 +18,7 @@ from __future__ import annotations
 
 from fabledassistant.mcp._context import current_user_id
 from fabledassistant.services import milestones as milestones_svc
+from fabledassistant.services import notes as notes_svc
 from fabledassistant.services import projects as projects_svc
 from fabledassistant.services import rulebooks as rulebooks_svc
 from fabledassistant.services import trash as trash_svc
@@ -34,6 +35,71 @@ async def list_projects() -> dict:
     return {"projects": [p.to_dict() for p in rows]}
 
 
+async def enter_project(project_id: int) -> dict:
+    """Session-start handshake: load full context for working on a project.
+
+    Call this FIRST whenever you're about to do project-scoped work
+    (start_planning, create_task, update_*, anything that takes a project_id).
+    One round-trip returns the project, its applicable rules (both rulebook-
+    subscribed and project-scoped), milestone progress, open tasks, and
+    recently-updated notes — everything you need to know the lay of the land
+    before mutating.
+
+    No persistent server state: this is a read snapshot. Re-call if the
+    session goes idle long enough that the data feels stale.
+
+    Args:
+        project_id: The project to enter.
+
+    Returns a dict with keys: project, milestone_summary, applicable_rules,
+    project_rules, subscribed_rulebooks, applicable_rules_truncated,
+    open_tasks, recent_notes.
+    """
+    uid = current_user_id()
+    project = await projects_svc.get_project(uid, project_id)
+    if project is None:
+        raise ValueError(f"project {project_id} not found")
+
+    applicable = await rulebooks_svc.get_applicable_rules(
+        project_id=project_id, user_id=uid,
+    )
+    milestone_summary = await milestones_svc.get_project_milestone_summary(
+        uid, project_id,
+    )
+    open_tasks, _ = await notes_svc.list_notes(
+        uid, is_task=True, project_id=project_id,
+        status=["todo", "in_progress"], sort="updated_at", limit=10,
+    )
+    recent_notes, _ = await notes_svc.list_notes(
+        uid, is_task=False, project_id=project_id,
+        sort="updated_at", limit=5,
+    )
+
+    return {
+        "project": project.to_dict(),
+        "milestone_summary": milestone_summary,
+        "applicable_rules": applicable["rules"],
+        "project_rules": applicable.get("project_rules", []),
+        "subscribed_rulebooks": applicable["subscribed_rulebooks"],
+        "applicable_rules_truncated": applicable["truncated"],
+        "open_tasks": [
+            {
+                "id": t.id, "title": t.title, "status": t.status,
+                "priority": t.priority, "task_kind": t.task_kind,
+                "milestone_id": t.milestone_id,
+            }
+            for t in open_tasks
+        ],
+        "recent_notes": [
+            {
+                "id": n.id, "title": n.title,
+                "updated_at": n.updated_at.isoformat() if n.updated_at else None,
+            }
+            for n in recent_notes
+        ],
+    }
+
+
 async def get_project(project_id: int) -> dict:
     """Fetch a Scribe project by ID.
 
@@ -137,6 +203,7 @@ async def delete_project(project_id: int) -> dict:
 def register(mcp) -> None:
     for fn in (
         list_projects,
+        enter_project,
         get_project,
         create_project,
         update_project,