docs: add AI pipeline hardening plan; rename giteaService to githubService

Adds AI_PIPELINE_HARDENING_PLAN.md — a phased, self-contained plan an AI
agent can execute to harden the Anthropic integration (central LLM
client, tool-based structured outputs, prompt caching, retrieval-based
R42 context, eval harness).

Renames src/lib/giteaService.js to src/lib/githubService.js. The module
calls api.github.com and raw.githubusercontent.com; the previous name
was misleading. No behaviour change. Updates the single import site in
src/components/admin/KnowledgeGraph.jsx.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/admin/KnowledgeGraph.jsx

@@ -4,7 +4,7 @@ import { Trash2, Edit2, Save, X, RefreshCw, AlertCircle, Plus, Link as LinkIcon
 import * as db from '../../lib/db';
 import { anthropicApi } from '../../lib/api';
 import { analyzeHandbookDelta } from '../../lib/extractionPipeline';
-import { getRepoFolder, getFileContent } from '../../lib/giteaService';
+import { getRepoFolder, getFileContent } from '../../lib/githubService';
 import Button from '../ui/Button';
 import SuggestionsQueue from './SuggestionsQueue';