fix: v0.5.18 P1 follow-ups — diff guard, prompt injection, docs, timeout, changelog

[anandgupta42]

Address the 5 P1 issues identified during the v0.5.18 multi-persona release evaluation:

1. GitLab MR diff size guard — large MRs hit context window with poor error
2. Prompt injection via MR content — MR title/desc/comments interpolated verbatim into LLM prompt
3. Stale GitLab docs — docs/usage/gitlab.md still says altimate github
4. validateCredentials missing timeout — TUI hangs on network partition
5. /login naming mismatch — changelog says /login but feature is provider dialog