feat: full file context + patterns-repo support

Major improvements to review quality:

1. Full file context: fetch complete content of all modified files from
   the PR branch and include as reference. This eliminates false-positive
   "missing import" findings since the model sees the entire file.

2. Patterns repo: new --patterns-repo / PATTERNS_REPO flag fetches
   language idiom files from a separate Gitea repo (e.g. rodin/elixir-patterns)
   and includes them as review criteria.

3. Multi-file patterns: --patterns-files / PATTERNS_FILES accepts
   comma-separated file paths to fetch from the patterns repo.

New API methods:
- GetPullRequestFiles: list changed files in a PR
- GetFileContentRef: fetch file content from a specific branch/ref

Prompt changes:
- BuildSystemPrompt now accepts (conventions, patterns)
- BuildUserPrompt now accepts fileContext parameter
- File context displayed before diff for model reference
- Patterns presented as "review criteria" in system prompt

Composite action updated with patterns-repo and patterns-files inputs.

.gitea/actions/review/action.yml

@@ -37,6 +37,14 @@ inputs:
     description: 'Path to conventions file in the repo (e.g. CLAUDE.md)'
     required: false
     default: ''
+  patterns-repo:
+    description: 'Repo with language patterns (e.g. rodin/elixir-patterns)'
+    required: false
+    default: ''
+  patterns-files:
+    description: 'Comma-separated file paths to fetch from patterns repo'
+    required: false
+    default: 'README.md'
   temperature:
     description: 'LLM temperature (0 = server default)'
     required: false
@@ -123,6 +131,8 @@ runs:
         LLM_API_KEY: ${{ inputs.llm-api-key }}
         LLM_MODEL: ${{ inputs.llm-model }}
         CONVENTIONS_FILE: ${{ inputs.conventions-file }}
+        PATTERNS_REPO: ${{ inputs.patterns-repo }}
+        PATTERNS_FILES: ${{ inputs.patterns-files }}
         LLM_TEMPERATURE: ${{ inputs.temperature }}
       run: |
         ARGS=""