refactor(fs): split filesystem seam into provider ctx.fs + policy ctx.fileContext

Implements the split-the-filesystem-seam RFC. ctx.fs shrinks to a text-storage
provider seam (resolve/stat/readText/streamText/writeText/editText with branded
FsTargetKey/FsVersion and an explicit FsWriteExpectation); the new
dsh-file-context package owns the model-facing policy (read windowing,
observed-state, write/edit freshness) as the concrete ctx.fileContext service.

Authorization is now freshness-based rather than full/partial view: a windowed
read records the file version and authorizes a later edit when the file is
unchanged, removing the dead-end where reading lines 100-150 of a large file
could not edit line 120. editText stays a provider primitive so version guard +
literal match + atomic rewrite remain one critical section, and the stale check
runs before matching so a stale edit reports FS_STALE_VERSION. tool-fs injects
fileContext, never reaching around to ctx.fs (the no-bypass contract).

packages/fs/tool-fs/package.json

@@ -32,6 +32,7 @@
   ],
   "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",
@@ -40,6 +41,7 @@
   },
   "devDependencies": {
     "@deepseek-ai/dsh-agent": "workspace:^",
+    "@deepseek-ai/dsh-file-context": "workspace:^",
     "@deepseek-ai/dsh-fs": "workspace:^",
     "@deepseek-ai/dsh-fs-local": "workspace:^",
     "@deepseek-ai/dsh-llm": "workspace:^",