docs(raw-api): document gitea_request, env vars and policy examples

Adds docs/raw-api.md (two-layer policy, sensitive denylist, env vars, write-mode
warning), links it from index and api-reference, documents RAW_API_ENABLED /
RAW_API_ALLOW_SENSITIVE in .env.example, and adds commented virtual-tool-name
deny examples to policy.yaml.

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

policy.yaml

@@ -4,5 +4,20 @@ defaults:
 
 tools:
   deny: []
+  # The generic `gitea_request` tool authorizes each call under a coarse virtual
+  # tool name of the form `gitea_request:<METHOD>:<top-path-segment>`, e.g.
+  # `gitea_request:GET:repos` or `gitea_request:DELETE:repos`. To keep raw
+  # dispatch read-only while still allowing GETs, deny the write methods here:
+  #
+  # deny:
+  #   - gitea_request:POST:repos
+  #   - gitea_request:PUT:repos
+  #   - gitea_request:PATCH:repos
+  #   - gitea_request:DELETE:repos
+  #
+  # NOTE: The admin/credential denylist (/admin, *tokens*, *secrets*, *hooks*,
+  # *keys*, applications/oauth2, runner registration tokens) is enforced in the
+  # handler independently of this file and is NOT configured here. It can only be
+  # overridden by setting RAW_API_ALLOW_SENSITIVE=true.
 
 repositories: {}