feat: audit-log Gitea MCP mutating actions with profile metadata (#18)

Add durable, opt-in audit logging for every mutating Gitea MCP action so an
operator can see which execution profile and authenticated Gitea user
performed (or was blocked from / failed) each mutation.

- New gitea_audit.py: pure, no-network module — recursive secret redaction
  (token/password/authorization keys; token/Basic/Bearer value runs),
  build_event (timestamp, action, result, profile, audit label, authenticated
  username, repo, issue/PR, target branch, head SHA, redacted request
  metadata), and an append-only JSON Lines sink.
- mcp_server.py: _audit helper + _audited context manager (simple mutations)
  and an _audit_pr_result decorator (gated review/merge tools, reading their
  own result dict) wired into create_issue, create_pr, edit_pr, close_issue,
  commit_files, delete_branch, create_label, set_issue_labels, mark_issue
  (label/unlabel), gitea_submit_pr_review, and gitea_merge_pr.
- Outcomes recorded as allowed/blocked/failed/succeeded; blocked and failed
  eligibility checks are logged, not just successes.

Off by default: records are written only when GITEA_AUDIT_LOG is set. When it
is unset every audit path short-circuits — no records, no extra API calls — so
existing tool behaviour and API call sequences are unchanged. Auditing never
raises; sink writes are best-effort. Tokens are never written.

Docs: README env table + audit note, .env.example placeholder.
Tests: tests/test_audit.py (19 cases) — redaction, event build, sink writes,
per-tool success/failure/blocked records, secret-free output, off-by-default
no-op, and audit-failure-never-breaks-action.

Closes #18

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

.env.example

@@ -30,6 +30,11 @@ GITEA_FORBIDDEN_OPERATIONS=merge,branch.push
 # Optional short label attached to this runtime for audit purposes.
 GITEA_AUDIT_LABEL=reviewer-runtime
 
+# Optional path to an audit log file (#18). When set, each mutating action
+# appends one redacted JSON record (profile + authenticated user + outcome).
+# Leave unset to disable auditing entirely (no records, no extra API calls).
+GITEA_AUDIT_LOG=/path/to/gitea-mcp-audit.log
+
 # Optional NAME of the token's source (e.g. an env var name). This is a name
 # only — never the token value. Surfaced by gitea_get_profile.
 GITEA_TOKEN_SOURCE=GITEA_TOKEN