cloud-tasks: support Forgejo/Gitea remotes in by-repo environment detection

Codex CLI currently misses PR/footer context for Forgejo/Gitea repos because by-repo environment detection is GitHub-only.

Observed behavior

• In repos with origin like https://git.home.invezt.io/<owner>/<repo> (Forgejo), Codex does not resolve repo context and PR/footer indicators are absent.
• Same workflow in github.com repos works.

Root cause

• codex-rs/cloud-tasks/src/env_detect.rs hardcodes GitHub parsing and provider routing:
• only parses github.com remote formats
• hardcodes .../by-repo/github/<owner>/<repo>

Proposed fix

• Parse generic repo refs from HTTPS + SSH forms into host/owner/repo.
• For non-GitHub hosts, try provider fallbacks in this order:

1) forgejo
2) gitea
3) github

• Preserve current GitHub behavior: host github.com uses only provider github.

Patch available

• Branch: tfiala:fix/forgejo-by-repo-provider
• Compare: https://github.com/openai/codex/compare/main...tfiala:fix/forgejo-by-repo-provider

What this patch changes

• Adds RepoRef { host, owner, repo } parser.
• Replaces GitHub-only parse_owner_repo logic with generic parse_repo_ref.
• Adds provider_candidates_for_host and uses it in both:
• autodetect_environment_id
• list_environments
• Adds unit tests for:
• GitHub HTTPS + SSH SCP parsing
• Forgejo HTTPS + SSH URL parsing
• invalid remote rejection
• provider-candidate selection

Validation run

• cargo test -p codex-cloud-tasks env_detect -- --nocapture
• Result: 7 env_detect tests passed.