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).

pnpm-lock.yaml

@@ -236,8 +236,23 @@ importers:
         specifier: ^4.0.0-rc.6
         version: 4.0.0-rc.6(@cordisjs/plugin-include@1.0.4)(@cordisjs/plugin-loader@1.0.0-rc.4)
 
+  packages/fs/file-context:
+    devDependencies:
+      '@deepseek-ai/dsh-fs':
+        specifier: workspace:^
+        version: link:../fs
+      '@deepseek-ai/dsh-llm':
+        specifier: workspace:^
+        version: link:../../llm/llm
+      cordis:
+        specifier: ^4.0.0-rc.6
+        version: 4.0.0-rc.6(@cordisjs/plugin-include@1.0.4)(@cordisjs/plugin-loader@1.0.0-rc.4)
+
   packages/fs/fs:
     devDependencies:
+      '@deepseek-ai/dsh-brand':
+        specifier: workspace:^
+        version: link:../../util/brand
       '@deepseek-ai/dsh-llm':
         specifier: workspace:^
         version: link:../../llm/llm
@@ -266,6 +281,9 @@ importers:
       '@deepseek-ai/dsh-agent':
         specifier: workspace:^
         version: link:../../core/agent
+      '@deepseek-ai/dsh-file-context':
+        specifier: workspace:^
+        version: link:../file-context
       '@deepseek-ai/dsh-fs':
         specifier: workspace:^
         version: link:../fs