feat: add read-only gitea_whoami authenticated-user lookup (#11)

Add a read-only MCP tool that calls Gitea's authenticated-user
endpoint (GET /api/v1/user) and returns safe identity metadata only:
username, display name, user id, email, server, and remote.

This lets future review/merge workflows prove which Gitea account the
MCP server is authenticated as, so self-review/self-merge can be
detected before acting — the blocker discovered during PR #8 dogfooding.

- Never returns the token, Authorization header, password, or secrets.
- Fails closed with a clear error if identity cannot be determined.
- No mutation; no profile switching; no review/approve/merge behavior.

Tests: identity mapping, secret-redaction, fail-closed, unknown-remote.

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

README.md

@@ -50,6 +50,7 @@ Any MCP-compatible agent (Antigravity, Claude Code, etc.) can call these tools n
 | `gitea_close_issue` | Close an issue by number |
 | `gitea_list_issues` | List issues with state/label filters |
 | `gitea_view_issue` | Get full details of a single issue |
+| `gitea_whoami` | Read-only: identify the authenticated Gitea account (safe metadata only) |
 | `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 |