CUST-WP-0074-T02: task wait qualifiers in the hub read model

Adds tasks.decision_id and workplan_dependencies.from_task_id (migration
e8f9a0b1c2d3). A from_task_id column is needed rather than
from_workplan_id plus description: the partial unique indexes key on
(from_workplan_id, target, relationship_type), so two tasks in one
workplan waiting on the same target could not both be indexed, and the
derived wait_kind needs per-task attribution. The two existing unique
indexes are narrowed to frontmatter edges (from_task_id IS NULL) and two
task-origin counterparts are added; downgrade deletes task-origin rows
before restoring the old indexes.

POST /workplans/{id}/dependencies/ accepts from_task_id (must belong to
the from workplan; a task cannot depend on itself). TaskCreate/Update/Read
carry decision_id.

Derived read-model fields, no new write routes (rule 5):
- TaskRead.wait_kind: external | human | both | unqualified, null unless
  status is wait (dependency rows from this task / needs_human).
- WorkplanRead.blocked_kind: human | external | none, null unless status
  is blocked. Human wins over external.
Both come from api/services/wait_kind.py, applied on GET /tasks/,
GET /tasks/{id}, GET /workplans/ and GET /workplans/{id}.

The identifier-migration reference-count proof now includes from_task_id
(FK count guard 22 -> 23).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: sonnet
Assistant-Process: 237582@bnt-lap001
Assistant-Session: f2b3d9f1-8fb9-4b9c-bc2b-837ec5dfc826

api/routers/tasks.py

@@ -20,6 +20,7 @@ from api.schemas.task import (
 )
 from api.services.legacy_compat import meter_legacy_body_from_model, meter_legacy_query_param
 from api.services.lifecycle import status_value, transition_task_status
+from api.services.wait_kind import annotate_tasks
 from api.task_status import normalize_task_status
 
 router = APIRouter(prefix="/tasks", tags=["tasks"])
@@ -69,7 +70,7 @@ async def list_tasks(
     if limit is not None:
         q = q.limit(limit)
     result = await session.execute(q)
-    return list(result.scalars().all())
+    return list(await annotate_tasks(session, list(result.scalars().all())))
 
 
 @router.get("/counts", response_model=list[TaskCountRead])
@@ -248,6 +249,7 @@ async def get_task(
     task = await session.get(Task, task_id)
     if task is None:
         raise HTTPException(status_code=404, detail="Task not found")
+    await annotate_tasks(session, [task])
     return task