feat: attempt ledger + resume preamble (v6)

Foundry 'platform restarts with original input' gap, after
reading the existing mechanisms: creation idempotency
(UNIQUE project_id+idempotency_key), goal checkpoints
(task_checkpoints per round, same-thread breakpoint resume
with no-progress fuse), merge/cost/notification CAS, and
coordinator child reuse already exist. The uncovered segment
is non-goal platform-interrupted tasks: auto_resume requeues
them fresh in the same worktree after clearing result/error/
interrupted_reason, so the new run gets only the original
prompt and cannot tell what the previous attempt had done.

This track adds, in worker/server layer only (no host.py or
codex thread change): task_attempts ledger, recorded by
auto_resume_interrupted in the same transaction before
clearing (attempt_no = old auto_resume_count + 1, kind
goal_breakpoint|platform_fresh, interrupted_reason,
checkpoint_round, result/error truncated to 2000 chars,
INSERT OR IGNORE keeps the first record on duplicate);
resume preamble injected in-memory only at claim for
platform_fresh reruns (previous attempt facts, truncated to
1000 chars, plus check-worktree-first and verify-before-
recreating-external-artifacts instructions; skipped for
coordinator/_resume/breakpoint tasks and when no record
exists, never fabricated); attempts surfaced read-only in
GET /tasks/{id}/timeline under existing project-read
permission. Fresh PG schema, migrations table list, and
server/worker ensure paths share one DDL.

Deliberately NOT claimed: the preamble is an instruction,
not a hard guarantee like creation idempotency or merge
CAS; the ledger does not track worktree files or external
artifacts (no worktree snapshot, executor-dependent paths;
hard dedup for those still rests on their existing gates);
manual retry paths do not go through auto_resume, so they
get no ledger record or preamble. Tests:
test_attempts.py (7 tests).

cloud/server.py

@@ -61,6 +61,7 @@ from cloud.agent_def import default_agents_dir as _agent_default_dir
 from cloud.agent_def import list_known_skills as _agent_known_skills
 from cloud.agent_def import parse_agent_definition as _parse_agent_definition
 from cloud.agent_def import resolve_registry_agent as _resolve_agent
+from cloud.attempts import ensure_attempts_table as _ensure_attempts
 from cloud.auth import ROLES as _MEMBER_ROLES
 from cloud.auth import AuthStore
 from cloud.coordinator import parse_parts as _coord_parse_parts
@@ -1717,6 +1718,7 @@ def init_db(db_path):
             _ensure_uploads_tables(conn)
             _ensure_task_checkpoints_table(conn)
             _ensure_task_steps_table(conn)
+            _ensure_attempts(conn)
             conn.commit()
             return
         conn.execute(
@@ -1753,6 +1755,7 @@ def init_db(db_path):
         _ensure_uploads_tables(conn)
         _ensure_task_checkpoints_table(conn)
         _ensure_task_steps_table(conn)
+        _ensure_attempts(conn)
         ensure_kill_requests_table(conn)
         # 常驻监听表（P0 S3 的 SQLite 侧）：启动时建一次，
         # 热路径（claim/run_due/批量）不再逐条 ensure，
@@ -5390,10 +5393,12 @@ class CloudControlPlaneHandler(BaseHTTPRequestHandler):
             "followups": [_followup_to_dict(r) for r in rows]})
 
     def _task_timeline(self, user_id, task_id):
-        """GET /tasks/{id}/timeline（P0 三件其三）：一个任务一张
-        时间线。steps 直读 task_steps（claim/resume/round/final），
-        checkpoints 直读 task_checkpoints（每轮断点快照）；旧
-        任务（部署前跑完、无步骤记录）返回空数组，不编造。
+        """GET /tasks/{id}/timeline（P0 三件其三 + v6 尝试账本）：
+        一个任务一张时间线。steps 直读 task_steps
+        （claim/resume/round/final），checkpoints 直读
+        task_checkpoints（每轮断点快照），attempts 直读
+        task_attempts（自动续跑前记下的正结束尝试）；旧任务
+        （部署前跑完、无步骤记录）返回空数组，不编造。
         """
         row = self._fetch_task(task_id)
         if row is None:
@@ -5409,6 +5414,7 @@ class CloudControlPlaneHandler(BaseHTTPRequestHandler):
         try:
             _ensure_task_checkpoints_table(conn)
             _ensure_task_steps_table(conn)
+            _ensure_attempts(conn)
             step_rows = conn.execute(
                 "SELECT step_no, kind, summary, state_sha, created_at"
                 " FROM task_steps WHERE task_id = ?"
@@ -5419,6 +5425,12 @@ class CloudControlPlaneHandler(BaseHTTPRequestHandler):
                 " verify_out, created_at FROM task_checkpoints"
                 " WHERE task_id = ? ORDER BY round_no",
                 (task_id,)).fetchall()
+            attempt_rows = conn.execute(
+                "SELECT attempt_no, kind, interrupted_reason,"
+                " checkpoint_round, result_text, error_text, created_at"
+                " FROM task_attempts WHERE task_id = ?"
+                " ORDER BY attempt_no",
+                (task_id,)).fetchall()
         finally:
             conn.close()
         self._send_json(200, {
@@ -5430,6 +5442,12 @@ class CloudControlPlaneHandler(BaseHTTPRequestHandler):
                              "goal_status": r[2], "state_sha": r[3],
                              "verify_out": r[4], "created_at": r[5]}
                             for r in cp_rows],
+            "attempts": [{"attempt_no": r[0], "kind": r[1],
+                          "interrupted_reason": r[2],
+                          "checkpoint_round": r[3],
+                          "result_text": r[4], "error_text": r[5],
+                          "created_at": r[6]}
+                         for r in attempt_rows],
         })
 
     def _task_files(self, user_id, task_id, query):