refactor(fs): make dsh-file-context an event-gate plugin, not a method service

Invert the tool↔policy control flow per the file-context event-gate RFC.
dsh-tool-fs becomes the executor — it reads/writes/edits through ctx.fs
directly, owns read windowing, and dispatches fs/write-expectation /
fs/edit-expectation (single-slot waterfalls) plus a contained fs/observed
emit. dsh-file-context drops its ctx.fileContext service and becomes a pure
event-gate plugin (observed-state + read-before-edit + version-guarded
write/edit, decided on those events). The provider's version guard becomes
optional so ctx.fs alone is a complete unconstrained text-storage seam:
removing the policy plugin gracefully loses the policy instead of breaking
the tool at a service-injection boundary.

packages/fs/tool-fs/package.json

@@ -32,7 +32,6 @@
   ],
   "license": "BSD-3-Clause",
   "peerDependencies": {
-    "@deepseek-ai/dsh-file-context": "^0.0.1",
     "@deepseek-ai/dsh-fs": "^0.0.1",
     "@deepseek-ai/dsh-llm": "^0.0.1",
     "@deepseek-ai/dsh-system-prompt": "^0.0.1",