feat(6854): the server says which project a pending act would change, and whether this session may do it

GET /api/plugin/write-scope answers the plugin's fence. The session's project
comes from repo / project_id, as every plugin endpoint reads it. The act is
either:
- a directory (target_repo / target_project_id), for a file edit or a git
  command; or
- an MCP call (tool, plus input: the call's short arguments).

It returns allowed, the other projects the act would change, and the reason
the operator is asked. It is a GET so a read-scoped key still fences file
edits and git.

services/write_scope.WRITE_TARGETS lists every write tool with the arguments
that locate its write: a project id, or a task, note, milestone, System, rule,
relation, trash batch, repo or family decision, each traced to its project. A
write that lands in no project (install settings, user-level records, the
judgments a session is asked for anywhere) has an empty entry. The one
cross-project write allowed is create_task(kind="issue") as a new, unplanned,
open issue. A project the caller cannot read is never named.

tests/test_write_scope.py:
- requires an entry for every write tool;
- requires every declared argument to exist on the tool;
- refuses a locating argument (project_id, task_id, milestone_id and the
  rest) that a write tool takes without declaring it.

The integration test traces each locator from real rows.

Milestone 583, step 2.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_integration_write_scope.py

@@ -0,0 +1,86 @@
+"""Real-Postgres proof that a write is traced to the project it lands in.
+
+Two projects of one user: the session's own and another. Each locator kind is
+resolved from a real row, and check_act answers for a session standing in
+the first.
+"""
+import pytest
+import pytest_asyncio
+from sqlalchemy import select
+
+from grimoire.models import async_session
+from grimoire.models.milestone import Milestone
+from grimoire.models.note import Note
+from grimoire.models.project import Project
+from grimoire.models.system import System
+from grimoire.services import write_scope
+from tests.helpers import ensure_user
+
+pytestmark = [pytest.mark.integration, pytest.mark.usefixtures("_dispose_engine")]
+
+OWNER = "write_scope_owner"
+BATCH = "write-scope-test-batch"
+
+
+@pytest_asyncio.fixture
+async def two_projects():
+    async with async_session() as s:
+        user = await ensure_user(s, OWNER)
+        for model in (Note, Milestone, System, Project):
+            for row in (await s.execute(select(model).where(model.user_id == user.id))).scalars():
+                await s.delete(row)
+        await s.flush()
+        home = Project(user_id=user.id, title="Home")
+        other = Project(user_id=user.id, title="Elsewhere")
+        s.add_all([home, other])
+        await s.flush()
+        plan = Milestone(user_id=user.id, project_id=other.id, title="Their plan")
+        area = System(user_id=user.id, project_id=other.id, name="Their area")
+        s.add_all([plan, area])
+        await s.flush()
+        theirs = Note(user_id=user.id, project_id=other.id, milestone_id=plan.id,
+                      title="Their task", body="", note_type="note", task_kind="work",
+                      status="todo")
+        mine = Note(user_id=user.id, project_id=home.id, title="My task", body="",
+                    note_type="note", task_kind="work", status="todo")
+        trashed = Note(user_id=user.id, project_id=other.id, title="Trashed", body="",
+                       note_type="note", deleted_batch_id=BATCH)
+        s.add_all([theirs, mine, trashed])
+        await s.commit()
+        return {"user": user.id, "home": home.id, "other": other.id, "plan": plan.id,
+                "area": area.id, "theirs": theirs.id, "mine": mine.id}
+
+
+async def test_each_locator_finds_the_project_of_the_row_it_names(two_projects):
+    ids = two_projects
+    other, home, uid = ids["other"], ids["home"], ids["user"]
+
+    async def lands(tool, **args):
+        return await write_scope.tool_targets(uid, tool, args)
+
+    assert await lands("add_task_log", task_id=ids["theirs"]) == {other}
+    assert await lands("update_task", task_id=ids["mine"]) == {home}
+    assert await lands("update_task", task_id=ids["mine"], milestone_id=ids["plan"]) == {home, other}
+    assert await lands("delete_milestone", milestone_id=ids["plan"]) == {other}
+    assert await lands("update_system", system_id=ids["area"]) == {other}
+    assert await lands("restore", deleted_batch_id=BATCH) == {other}
+    assert await lands("create_rule", topic_id=1) == set()
+
+
+async def test_a_session_in_one_project_is_asked_before_writing_in_the_other(two_projects):
+    ids = two_projects
+    uid, home, other = ids["user"], ids["home"], ids["other"]
+
+    own = await write_scope.check_act(uid, home, tool="add_task_log",
+                                      args={"task_id": ids["mine"]})
+    assert own["allowed"] is True
+
+    theirs = await write_scope.check_act(uid, home, tool="add_task_log",
+                                         args={"task_id": ids["theirs"]})
+    assert theirs["allowed"] is False
+    assert theirs["targets"] == [{"id": other, "title": "Elsewhere"}]
+    assert "Home" in theirs["reason"] and "Elsewhere" in theirs["reason"]
+
+    issue = await write_scope.check_act(uid, home, tool="create_task",
+                                        args={"project_id": other, "kind": "issue"})
+    assert issue["allowed"] is True and issue["issue"] is True