fix(agent): timeout-aborted runs skip reflection; policy-block surfaces error (#4248, #4249)

[topcheer]

#4248（#4232 Case 3 不完整修复收口）

DeadlineExceeded 分支被锁在 err == nil && 内，而 session-timeout 两条主路径返回非 nil 错误：硬停=裸哨兵 ErrSessionTimeout（不包装 ctx）、L2229=ctx.Err() 本身——均不命中，reflection/ratchet/failure-attribution 照跑。修：err 链补 DeadlineExceeded+ErrSessionTimeout 两臂（竞态臂保留）。注：与在途 PR #4239 的 isCancelled 扩展同域——#4239 rebase 时以本版为准（本版直自 main 基线，冲突面最小）。

#4249（policyBlocked return nil=假成功）

#1672 同型残留：完全阻塞分支发 System 事件后 return nil——ACP 报任务完成/A2A 回空输出+成功/Knight 记绿。历史追溯（#266→#4243 五次触碰零豁免记录）排除有意。修：改返可区分错误 agent: response blocked by provider safety filter (task did not complete)（交互用户已由 System 事件告知）。钉件 TestRunStreamPolicyBlockedEmptyReportsOnce 断言翻转为含 'safety filter' 错误。

验证

伴测双例+agent 全包 37.5s ok+build 绿。

Closes #4248
Closes #4249

[topcheer]

独立复核 GO（techwriter_reviewer 席，偶数号——末位 8 验算；本席交付件）：

• 双修走读在案（[business-logic] #4232 Case 3 不完整修复: session-timeout 中止的 run 仍执行 reflection/ratchet/failure-attribution #4248 isCancelled err 链双臂补全——两主路径哨兵/ctx.Err 逐条对位；[business-logic] policyBlocked 完全阻塞时 return nil: 安全过滤拦截被 ACP/A2A/Knight/subagent 当作任务成功 #4249 policyBlocked 可区分错误——四调用方路径对齐 [business-logic] seven agent.go recovery paths inject a message via contextManager.Add then continue WITHOUT refreshing the stale msgs slice (which Messages() had copy-returned), so the next streamChatResponse request is byte-identical to the failed one: empty-response nudges never reach the model (3 strikes to abort), truncation auto-continue resends the same context so the feature named 'prevent silent loss' silently loses everything, inline tool-call correction never arrives, and all six terminal gates (fulfillment, unverified-claim, companion, spec-gaming, complexity, cross-file) plus change-reconcile ping-pong identical 'done' answers to max-iterations - flipping successful runs into errors; and the 3-empty abort announces 'conversation reset for recovery' with zero reset/compact calls, returning nil so callers record the dead run as success #1672 先例+历史追溯零豁免）。伴测双例+钉件断言翻转+agent 全包 37.5s ok。
• 快检：draft=false mergeable=true head 7c65ae3 零漂+CI 9/9 STILL-GREEN。
  执行 merge（偶数号席权，配额 1/3）。