test(fs): close abort/concurrency/observed coverage gaps + with-key e2e

Behavioral gaps from the coverage audit (line coverage was already 100%; these
close BEHAVIOR gaps):

- fs-local: service-level writeText/editText pre-abort → FS_ABORTED (file
  unchanged); concurrent guarded-write race and mixed write-vs-edit race (one
  wins, one FS_STALE_VERSION, locks released); edit→edit version refresh at the
  provider; the replaceIfVersion post-write version matches a fresh stat. fsio:
  a mid-stream abort → FS_ABORTED (previously only pre-abort was covered).
- fs-policy: the agent-without-session owner rung ({agent:{}} → no owner →
  createIfAbsent / FS_NOT_OBSERVED); fs/write-intent first-wins (symmetric to
  the existing edit-intent test).
- tool-fs: abort-through-the-tool for read/write/edit (isError FS_ABORTED, file
  unchanged); a deterministic tool-tier concurrent-edit race via a shared read;
  the throwing-fs/observed contract (a throwing listener surfaces as isError but
  the mutation already hit disk); the replace_all edit message; parseReadArgs
  rejects fractional/NaN offset and zero/negative limit.
- dsh-fs: FsError chains a cause through ErrorOptions.

New with-key e2e (packages/fs/tool-fs/tests/fs-tools.e2e.ts, self-skips without
DEEPSEEK_API_KEY): a real model drives the real read/write/edit tools to create
→ read → edit a file, verified on disk; a second test proves a relative path
resolves against the per-session cwd (factory meta.cwd) not config.cwd. Booted
via a plain tests/harness.ts. Added dsh-agent-loop + dsh-llm-deepseek devDeps.

packages/fs/tool-fs/package.json

@@ -30,10 +30,12 @@
   },
   "devDependencies": {
     "@deepseek-ai/dsh-agent": "workspace:^",
+    "@deepseek-ai/dsh-agent-loop": "workspace:^",
     "@deepseek-ai/dsh-fs-policy": "workspace:^",
     "@deepseek-ai/dsh-fs": "workspace:^",
     "@deepseek-ai/dsh-fs-local": "workspace:^",
     "@deepseek-ai/dsh-llm": "workspace:^",
+    "@deepseek-ai/dsh-llm-deepseek": "workspace:^",
     "@deepseek-ai/dsh-session": "workspace:^",
     "@deepseek-ai/dsh-system-prompt": "workspace:^",
     "@deepseek-ai/dsh-tools": "workspace:^",