fix: address codex review round 2

- edit tool: add the read-before-edit requirement to the model-facing prompt
  (with the just-created/edited-this-session exception), matching write's
  guidance so the model doesn't only learn it via a failed FS_NOT_OBSERVED call.
- fsspec-style-fs-seam RFC: correct the acceptance criteria that still claimed
  a ctx.fileContext service and a fileContext inject — the landed design is the
  fs/* event gate with the tool injecting fs.
- filesystem-tool-schemas RFC: replace the stale "prior full file state"
  edit requirement with version-freshness wording (any windowed read authorizes
  a fresh edit; no partial-view flag).

packages/fs/tool-fs/src/edit.ts

@@ -54,7 +54,7 @@ export function apply(ctx: Context): void {
   ctx.systemPrompt.section({
     name: 'tool:edit',
     order: 102,
-    text: 'Use the edit tool for targeted changes to existing UTF-8 text files. It replaces literal old_string with new_string; by default old_string must appear exactly once. If old_string appears multiple times, provide a more specific old_string or set replace_all to true.',
+    text: 'Use the edit tool for targeted changes to existing UTF-8 text files. It replaces literal old_string with new_string; by default old_string must appear exactly once. If old_string appears multiple times, provide a more specific old_string or set replace_all to true. Read the file first (the default file-context policy requires it), unless you just created or edited it in this session.',
   })
 
   ctx.tools.register(defineTool({