slack: use uucp for sanitization, harden redirects/permissions/timeouts
- Replace hardcoded codepoint list with Uucp.Gc.general_category (Cf, Cc)
so sanitizer tracks the Unicode standard automatically
- Disable follow_redirects on OAuth token exchange POST to prevent
credential-bearing body from being forwarded on 3xx
- Enforce 0o600 via Unix.chmod after every secret/cache file save,
not just on creation
- Sanitize Block.pp JSON output
- Add 5s per-connection timeout on OAuth callback listener