MUL-5625: WS-2867: bound daemon workspace cache growth

[tangyuanjc]

Summary

• create and self-heal .metadata_never_index at daemon workspace roots
• treat codex-home/.tmp as an exact managed artifact alongside .sandbox-bin
• reclaim task-local Codex caches after the provider exits and before terminal reporting, while retaining sessions, logs, workdir, output, auth, and config
• extend GC/disk-usage coverage and document the resulting lifecycle

Safety and scope

• cleanup continues to reject symlinks and preserves same-named directories inside user repositories unless operators explicitly configure broad basename matching
• whole task roots retain the existing issue/orphan TTL behavior, so issue continuation and session resume are unchanged
• the live c37d6ffd / WS-2512 data/runs outlier is application-owned scheduled output, not runtime cache; this PR intentionally does not delete it or add a generic size cap

Test plan

• go test -count=1 -parallel=2 ./internal/daemon/execenv ./internal/daemon
• git diff --check origin/main...HEAD

Closes WS-2867

[vercel]

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[multica-eve]

这里有一个合并前必须修复的文件安全问题：server/internal/daemon/execenv/spotlight.go:23 直接调用 os.WriteFile 刷新 .metadata_never_index。如果该路径被先前任务替换成符号链接，下一次 Prepare / Reuse 会跟随链接并截断工作区外的目标文件。

请改为不跟随现有符号链接的安全写入。建议沿用仓库现有根标记的“同目录临时文件 + rename”原子替换模式，让符号链接本身被替换为普通空文件；至少也应先用 Lstat 拒绝非普通文件。

请同时补一个回归测试：在工作区外创建带内容的哨兵文件，把 .metadata_never_index 做成指向它的符号链接，然后调用 EnsureSpotlightExclusion；断言哨兵内容未改变，并验证标记被安全替换为普通空文件（若选择拒绝策略，则验证安全报错且目标未改变）。