fix: use os.pathsep, add tests, update tips for multi-root support

- Use os.pathsep instead of literal ':' so Windows paths (C:\dir) and
  the Windows separator ';' work correctly.
- Add 9 tests covering multi-root behavior: writes inside first/second
  root, writes outside all roots, trailing/leading/double separators,
  all-separators edge case, static deny priority, duplicate dedup.
- Update hermes_cli/tips.py tip string to mention multiple paths.
- Update docs to mention os.pathsep / ; on Windows.

Follow-up for salvaged PR #49557.

hermes_cli/tips.py

@@ -389,7 +389,7 @@ TIPS = [
     # --- Env Vars & Config Gates ---
     "display.tool_progress_command: true exposes /verbose on messaging platforms; it's CLI-only by default.",
     'HERMES_BACKGROUND_NOTIFICATIONS=result only pings when background tasks finish (vs all/error/off).',
-    'HERMES_WRITE_SAFE_ROOT restricts write_file and patch to a directory prefix; writes outside require approval.',
+    'HERMES_WRITE_SAFE_ROOT restricts write_file/patch to directory prefixes; multiple paths via os.pathsep (: or ;).',
     'HERMES_IGNORE_RULES skips auto-injection of AGENTS.md, SOUL.md, .cursorrules, memory, and preloaded skills.',
     'HERMES_ACCEPT_HOOKS auto-approves unseen shell hooks declared in config.yaml without a TTY prompt.',
     'auxiliary.goal_judge.model routes the /goal judge to a cheap fast model to keep loop cost near zero.',