feat: add read-only gitea_check_pr_eligibility (#14)

Add a read-only MCP tool that decides whether the current authenticated
identity + active runtime profile is eligible to review, approve,
request_changes, or merge a specific PR. Evaluation only — it never
reviews, approves, requests changes, merges, or mutates anything.

Inspects: authenticated username (/user), active profile metadata
(allowed/forbidden operations), and PR facts (author, state, head SHA,
mergeability). Returns {eligible, requested_action, authenticated_user,
profile_name, pr_author, pr_state, head_sha, mergeable, reasons}.

Fail-closed rules:
- unknown action / unknown remote -> not eligible
- action not in allowed ops, or in forbidden ops -> not eligible
- identity undetermined -> not eligible
- authenticated user == PR author -> cannot approve/merge
- PR not open -> not eligible
- merge requires a positive mergeable signal

No token/auth-header exposure. No review/approve/request-changes
mutation. No merge mutation. No multi-token switching. No
Jenkins/Ops/GlitchTip/Release/deploy behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -52,6 +52,7 @@ Any MCP-compatible agent (Antigravity, Claude Code, etc.) can call these tools n
 | `gitea_view_issue` | Get full details of a single issue |
 | `gitea_whoami` | Read-only: identify the authenticated Gitea account (safe metadata only) |
 | `gitea_get_profile` | Read-only: describe the active runtime execution profile (safe metadata only) |
+| `gitea_check_pr_eligibility` | Read-only: check if the current identity/profile may review/approve/request_changes/merge a PR |
 | `gitea_mark_issue` | Claim/release an issue (start/done) |
 | `gitea_list_labels` | List all available labels in a repository |
 | `gitea_create_label` | Create a new label with custom color |