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>

tests/test_mcp_tool_projects.py

@@ -6,7 +6,7 @@ import pytest
 from fabledassistant.mcp._context import _user_id_ctx
 from fabledassistant.mcp.tools.projects import (
     list_projects, get_project, create_project,
-    update_project,
+    update_project, enter_project,
 )
 
 
@@ -130,3 +130,75 @@ async def test_update_project_raises_when_not_found():
     ):
         with pytest.raises(ValueError, match="project 999 not found"):
             await update_project(project_id=999, title="x")
+
+
+@pytest.mark.asyncio
+async def test_enter_project_composes_full_context():
+    """enter_project pulls project + rules + milestone summary + open tasks +
+    recent notes in one composed call."""
+    p = _fake_project(id=5, title="P")
+    applicable_payload = {
+        "rules": [{"id": 1, "title": "r1", "statement": "s",
+                   "topic_title": "t", "rulebook_title": "rb"}],
+        "project_rules": [{"id": 99, "title": "pr1", "statement": "ps"}],
+        "truncated": False,
+        "subscribed_rulebooks": [{"id": 2, "title": "rb"}],
+    }
+    milestone_summary = [{"id": 10, "title": "MS", "task_count": 3}]
+
+    task1 = MagicMock()
+    task1.id = 100; task1.title = "T1"; task1.status = "in_progress"
+    task1.priority = "high"; task1.task_kind = "work"; task1.milestone_id = 10
+    note1 = MagicMock()
+    note1.id = 200; note1.title = "N1"
+    note1.updated_at = None  # avoids datetime mocking
+
+    with patch(
+        "fabledassistant.mcp.tools.projects.projects_svc.get_project",
+        AsyncMock(return_value=p),
+    ), patch(
+        "fabledassistant.mcp.tools.projects.rulebooks_svc.get_applicable_rules",
+        AsyncMock(return_value=applicable_payload),
+    ), patch(
+        "fabledassistant.mcp.tools.projects.milestones_svc.get_project_milestone_summary",
+        AsyncMock(return_value=milestone_summary),
+    ), patch(
+        "fabledassistant.mcp.tools.projects.notes_svc.list_notes",
+        AsyncMock(side_effect=[([task1], 1), ([note1], 1)]),
+    ):
+        out = await enter_project(project_id=5)
+
+    assert out["project"]["id"] == 5
+    assert out["milestone_summary"] == milestone_summary
+    assert out["applicable_rules"][0]["title"] == "r1"
+    assert out["project_rules"][0]["id"] == 99
+    assert out["subscribed_rulebooks"] == [{"id": 2, "title": "rb"}]
+    assert out["open_tasks"][0]["id"] == 100
+    assert out["open_tasks"][0]["status"] == "in_progress"
+    assert out["recent_notes"][0]["id"] == 200
+
+
+@pytest.mark.asyncio
+async def test_enter_project_raises_when_project_not_found():
+    with patch(
+        "fabledassistant.mcp.tools.projects.projects_svc.get_project",
+        AsyncMock(return_value=None),
+    ):
+        with pytest.raises(ValueError, match="project 999 not found"):
+            await enter_project(project_id=999)
+
+
+def test_enter_project_registered_in_register():
+    """register(mcp) registers enter_project alongside the existing tools."""
+    from fabledassistant.mcp.tools.projects import register
+    registered: list[str] = []
+
+    class FakeMCP:
+        def tool(self, name=None):
+            def decorator(fn):
+                registered.append(name)
+                return fn
+            return decorator
+
+    register(FakeMCP())
+    assert "enter_project" in registered