feat: block direct MCP imports and unsanctioned keychain access (Closes #558)

Require a sanctioned MCP daemon (or pytest) before get_auth_header and
keychain credential fill so raw shell imports cannot bypass preflight gates.

docs/mcp-daemon-import-guard.md

@@ -0,0 +1,23 @@
+# MCP daemon import and keychain guard (#558)
+
+## Problem
+
+During deadlock debugging, agents imported `gitea_mcp_server` / ran credential
+helpers from a raw shell, bypassing preflight purity and role gates.
+
+## Rule
+
+Mutation auth and keychain fill require a **sanctioned MCP daemon** process.
+
+| Context | Allowed |
+|---------|---------|
+| Official MCP entrypoint (`mcp_server.py` / `gitea_mcp_server` `__main__`) sets `GITEA_MCP_SANCTIONED_DAEMON=1` | yes |
+| pytest | yes |
+| `GITEA_ALLOW_DIRECT_MCP_IMPORT=1` (operator/tests only) | yes |
+| bare `python -c 'import gitea_auth; get_auth_header(...)'` | **no** |
+| keychain fill without daemon | **no** unless `GITEA_ALLOW_KEYCHAIN_CLI=1` |
+
+## Operator note
+
+LLM sessions must never set the allow-direct-import or allow-keychain-cli
+overrides. Those are human-only escape hatches.