Cloud dispatch pool + auto PR (opt-in, skip clearly)

Worker pool: workers table + register/heartbeat (heartbeat is
registration, no 404), GET /workers with alive (120s default),
POST /workers/heartbeat (X-API-Key, missing worker_id 400).
Tasks gain executor (local/docker/cloud, None = any worker);
worker claim filters by executor in pick_next and in the CAS
WHERE clause, CAS semantics unchanged; CloudExecutor _name
now correctly returns cloud, not local.
CloudExecutor: generic HTTP runner interface (endpoint from
explicit arg or CLOUD_RUNNER_ENDPOINT, auth header name only,
token value only from env into the request header, never into
logs/body/errors); no endpoint configured = explicit failure,
never silent downgrade. Vendor adapters (ECS/Cloud Run) are
out of scope, documented as such.
Auto PR: projects.auto_pr (0/1, create-time only, no PATCH
route exists); POST /tasks/{id}/pr (maintainer+ via
approve_merge) only for merged tasks (else 409); skip reasons
in order: auto_pr_disabled, no_worktree, no_remote, no_gh,
gh_not_authenticated — all return 200 with pr_skipped, task
stays merged, no push attempted; push/pr-create failure
returns 502 with step, task stays merged; success writes
pr_url/pr_number and audits pr_created. No default auto-push
from the worker. Remote URLs in errors are scrubbed of
userinfo; gh credentials are managed by gh itself, never
stored by the control plane.
Tests: host 27 pass, cloud 323 pass (8 skipped are
environment-dependent real-Postgres tests).

cloud/db.py

@@ -781,7 +781,8 @@ POSTGRES_SCHEMA = (
     "name TEXT, "
     "owner_id TEXT, "
     "created_at DOUBLE PRECISION, "
-    "repo_url TEXT)",
+    "repo_url TEXT, "
+    "auto_pr INTEGER)",
     # tasks 含 server.migrate_tasks_table 补的那 6 列，建表时一次建齐；
     # 老库迁移仍走 migrate_tasks_table（PRAGMA 翻译后在 PG 上是 no-op）。
     "CREATE TABLE IF NOT EXISTS tasks("
@@ -802,6 +803,9 @@ POSTGRES_SCHEMA = (
     "started_at DOUBLE PRECISION, "
     "finished_at DOUBLE PRECISION, "
     "child_pid BIGINT, "
+    "executor TEXT, "
+    "pr_url TEXT, "
+    "pr_number INTEGER, "
     "UNIQUE(project_id, idempotency_key))",
     "CREATE TABLE IF NOT EXISTS summaries("
     "id BIGINT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY, "
@@ -875,6 +879,15 @@ def init_postgres_schema(conn):
         "PRAGMA table_info(projects)").fetchall()}
     if "repo_url" not in cols:
         conn.execute("ALTER TABLE projects ADD COLUMN repo_url TEXT")
+    if "auto_pr" not in cols:
+        conn.execute("ALTER TABLE projects ADD COLUMN auto_pr INTEGER")
+    tcols = {row[1] for row in conn.execute(
+        "PRAGMA table_info(tasks)").fetchall()}
+    for _name, _type in (("executor", "TEXT"), ("pr_url", "TEXT"),
+                         ("pr_number", "INTEGER")):
+        if _name not in tcols:
+            conn.execute(
+                "ALTER TABLE tasks ADD COLUMN %s %s" % (_name, _type))
     conn.commit()