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/schemas/task.py

@@ -45,6 +45,7 @@ class TaskCreate(TaskStatusMixin, TaskFlavorMixin, WorkplanIdCreateMixin):
     blocking_reason: str | None = None
     needs_human: bool = False
     intervention_note: str | None = None
+    decision_id: str | None = None
     parent_task_id: uuid.UUID | None = None
 
     @model_validator(mode="after")
@@ -65,6 +66,7 @@ class TaskUpdate(TaskStatusMixin, TaskFlavorMixin):
     blocking_reason: str | None = None
     needs_human: bool | None = None
     intervention_note: str | None = None
+    decision_id: str | None = None
     parent_task_id: uuid.UUID | None = None
     # Token passthrough — three tiers (highest precision wins):
     # 1. tokens_in + tokens_out  → exact counts; note defaults to "measured"
@@ -128,7 +130,12 @@ class TaskRead(TaskStatusMixin, WorkplanIdCompatMixin):
     blocking_reason: str | None = None
     needs_human: bool
     intervention_note: str | None = None
+    decision_id: str | None = None
     parent_task_id: uuid.UUID | None = None
+    # Derived (CUST-WP-0074 rule 5): "external" | "human" | "both" |
+    # "unqualified" while status is wait, else None. Set by
+    # api.services.wait_kind.annotate_tasks on read routes.
+    wait_kind: str | None = None
     created_at: datetime
     updated_at: datetime
 

api/schemas/workplan.py

@@ -155,6 +155,10 @@ class WorkplanRead(WorkplanStatusMixin):
     backing_relative_path: str | None = None
     backing_archived: bool | None = None
     backing_synced_at: datetime | None = None
+    # Derived (CUST-WP-0074 rule 5): "human" | "external" | "none" while
+    # status is blocked, else None. Set by
+    # api.services.wait_kind.annotate_workplans on read routes.
+    blocked_kind: str | None = None
     created_at: datetime
     updated_at: datetime
 

api/schemas/workplan_dependency.py

@@ -10,6 +10,7 @@ class WorkplanDependencyCreate(BaseModel):
         validation_alias=AliasChoices("to_workplan_id", "to_workstream_id"),
     )
     to_task_id: uuid.UUID | None = None
+    from_task_id: uuid.UUID | None = None
     relationship_type: str = "blocks"
     description: str | None = None
 
@@ -18,6 +19,7 @@ class WorkplanDependencyRead(BaseModel):
     model_config = ConfigDict(from_attributes=True)
     id: uuid.UUID
     from_workplan_id: uuid.UUID
+    from_task_id: uuid.UUID | None = None
     to_workplan_id: uuid.UUID | None = None
     to_task_id: uuid.UUID | None = None
     relationship_type: str