feat(#143): fetch doc-map config from trusted VCS ref

The doc-map YAML config was previously read from the local workspace
(the PR branch checkout). A malicious PR author could modify
.review-bot/doc-map.yml to map any path glob to sensitive design docs,
causing review-bot to fetch and inject those docs into the LLM prompt.

Fix: add --doc-map-trusted-ref (DOC_MAP_TRUSTED_REF) flag. When set to
a trusted ref (e.g. 'main'), the doc-map config is fetched from the VCS
API at that ref instead of from local workspace. A 404 from VCS is a
hard error (no silent fallback to local copy).

When unset, the local workspace is used with a security warning in the
logs pointing operators to the new flag.

Changes:
- review/docmap.go: add ParseDocMapConfigContent + parseDocMapBytes
  helper to parse from in-memory content (fetched via VCS API)
- cmd/review-bot/main.go: add --doc-map-trusted-ref flag; Step 6c
  branches on trusted-ref to fetch vs local-workspace load
- .gitea/actions/review/action.yml: add doc-map-trusted-ref input
- README.md: document new input
- CHANGELOG.md: security and feature entries

Tests:
- TestParseDocMapConfigContent_Valid/Empty/InvalidYAML/UnknownKeys
  in review/docmap_test.go

Coverage: 53.0% cmd/review-bot

.gitea/actions/review/action.yml

@@ -141,6 +141,16 @@ inputs:
     description: 'Maximum bytes of injected doc content from doc-map (default 102400 = 100KB)'
     required: false
     default: '102400'
+  doc-map-trusted-ref:
+    description: >-
+      Git ref (branch, tag, or SHA) from which to fetch the doc-map config file
+      via VCS API instead of reading it from the local workspace. Recommended
+      when using doc-map: set this to the default branch (e.g. 'main') so a
+      malicious PR cannot modify the doc-map config to inject arbitrary design
+      docs into the LLM prompt. When unset, the config is read from the local
+      workspace (the PR branch) with a security warning in the logs.
+    required: false
+    default: ''
 
 runs:
   using: 'composite'
@@ -507,6 +517,7 @@ runs:
         PERSONA_FILE: ${{ inputs.persona-file }}
         DOC_MAP_FILE: ${{ inputs.doc-map }}
         DOC_MAP_MAX_BYTES: ${{ inputs.doc-map-max-bytes }}
+        DOC_MAP_TRUSTED_REF: ${{ inputs.doc-map-trusted-ref }}
         AICORE_CLIENT_ID: ${{ inputs.aicore-client-id }}
         AICORE_CLIENT_SECRET: ${{ inputs.aicore-client-secret }}
         AICORE_AUTH_URL: ${{ inputs.aicore-auth-url }}