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/db.py

@@ -831,6 +831,18 @@ POSTGRES_SCHEMA = (
     "created_at DOUBLE PRECISION)",
     "CREATE INDEX IF NOT EXISTS idx_task_steps_task"
     " ON task_steps(task_id, step_no)",
+    # 尝试账本（v6，见 cloud/attempts.py）：(task_id, attempt_no)
+    # 唯一；PG 新库一次建齐，老库走 server/worker ensure。
+    "CREATE TABLE IF NOT EXISTS task_attempts("
+    "task_id TEXT, "
+    "attempt_no INTEGER, "
+    "kind TEXT, "
+    "interrupted_reason TEXT, "
+    "checkpoint_round INTEGER, "
+    "result_text TEXT, "
+    "error_text TEXT, "
+    "created_at DOUBLE PRECISION, "
+    "PRIMARY KEY(task_id, attempt_no))",
     "CREATE TABLE IF NOT EXISTS projects("
     "id TEXT PRIMARY KEY, "
     "name TEXT, "