ci: take bash and tools from the .#ci dev shell; stop policing versions

This follows the user's rulings: CI should not care which bash or which
standard utilities it runs, and jobs take what the ci dev shell provides.

- checkers.sh: remove find_modern_bash, the 5.1 floor (improvise-fd0j) and
  exit 3. Suites run under $BASH with PATH unchanged.
- checkers-test.sh: remove sections 2, 5 and 5b (the fake bashes and utilbin).
  They existed only to pin the floor.
- scripts/kb: drop the BSD `stat -f` fallback. kb needs the dev shell for tpl,
  and that shell's stat is GNU.
- ci.yml: remove every `shell: bash` override, so every step runs in the dev
  shell. Remove the web-toolchain version probe and the Playwright
  version/browser probe. The shell's PLAYWRIGHT_BROWSERS_PATH pins the
  browsers. npm ci stays because @playwright/test is not nix-provided.
- flake.nix: update the comments that promised those CI checks.

Reasoning: context/code/tooling.md section 5. Evidence:
context/test-evidence/tooling.md.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

scripts/kb

@@ -87,7 +87,7 @@ ledger_path() {
 # first line; bytes alone are not enough, since a restore can reproduce them.
 file_stamp() {
   printf '%s %s' "$(stamp_of "$1")" \
-    "$(stat -c '%i %.9Y' "$1" 2>/dev/null || stat -f '%i %Fm' "$1")"
+    "$(stat -c '%i %.9Y' "$1")"
 }
 ledger_holds() { [ -f "$1" ] && [ "$(head -n 1 "$1")" = "% kb-stamp $(file_stamp "$2")" ]; }