docs: post-ship sync for v1.38.0.0

Document the two architectural invariants that landed in v1.38.0.0 in
their persistent homes (not just CHANGELOG):

- README Windows section: add the `./setup` re-run-after-git-pull
  requirement that `_print_windows_copy_note_once` shows at runtime.
- CONTRIBUTING "Things to know": add the no-raw-`ln` invariant for
  contributors editing `setup`, with the test that enforces it.
- ARCHITECTURE: new "Unicode sanitization at server egress" section
  between Shell injection prevention and Prompt injection defense,
  with egress table (HTTP/batch/SSE) and the post-stringify-regex
  rationale.
- CLAUDE.md: cross-references for both invariants, matching the
  v1.6.0.0 dual-listener pattern (each constraint says which files
  to read before editing and which test pins it).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -459,6 +459,8 @@ Data is stored in [Supabase](https://supabase.com) (open source Firebase alterna
 
 **Windows users:** gstack works on Windows 11 via Git Bash or WSL. Node.js is required in addition to Bun — Bun has a known bug with Playwright's pipe transport on Windows ([bun#4253](https://github.com/oven-sh/bun/issues/4253)). The browse server automatically falls back to Node.js. Make sure both `bun` and `node` are on your PATH.
 
+On Windows without Developer Mode (MSYS2 / Git Bash), `setup` falls back to file copies instead of symlinks because `ln -snf` produces frozen copies that don't refresh on `git pull`. **Re-run `cd ~/.claude/skills/gstack && ./setup` after every `git pull`** so your skill files match the repo. `setup` prints a one-line note reminding you. Unix and WSL keep symlinks and don't need the re-run.
+
 **Claude says it can't see the skills?** Make sure your project's `CLAUDE.md` has a gstack section. Add this:
 
 ```