feat: implement incremental indexing and remove proactive rate limit

Major performance improvements for CI builds:

1. **Removed proactive rate limit threshold**
   - No longer waits at 500 remaining requests
   - Uses full 5000 request quota before forced wait
   - Maximizes work per rate limit cycle

2. **Implemented incremental indexing**
   - Checks if repository already exists in database
   - Compares last_commit (pushedAt) to detect changes
   - Only fetches README for new or updated repositories
   - Skips README fetch for unchanged repos (major time savings)

3. **Increased timeout to GitHub maximum**
   - Job timeout: 180m → 360m (6 hours, GitHub free tier max)
   - Script timeout: 170m → 350m
   - Allows full first-run indexing to complete

Impact on performance:

**First run (empty database):**
- Same as before: ~25,000 repos need full indexing
- Will use all 360 minutes but should complete

**Subsequent runs (incremental):**
- Only fetches READMEs for changed repos (~5-10% typically)
- Dramatically faster: estimated 30-60 minutes instead of 360
- Makes daily automated builds sustainable

Files changed:
- lib/github-api.js: Removed proactive rate limit check
- lib/indexer.js: Added incremental indexing logic
- .github/workflows/build-database.yml: Increased timeout to 360m

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

lib/indexer.js

@@ -261,6 +261,9 @@ async function buildIndex(force = false, mode = null) {
       // Index repositories
       for (const repo of repos) {
         try {
+          // Check if repo already exists (incremental indexing)
+          const existingRepo = db.getRepositoryByUrl(repo.url);
+
           // Get repo info from GitHub
           const repoInfo = await github.getRepoInfo(repo.url);
 
@@ -268,8 +271,20 @@ async function buildIndex(force = false, mode = null) {
             const repoId = db.addRepository(listId, repoInfo.name, repo.url, repo.description || repoInfo.description, repoInfo);
             indexedRepos++;
 
-            // Index README if in full mode
+            // Determine if we need to fetch README
+            let shouldFetchReadme = false;
             if (indexChoice === 'full' || indexChoice === 'sample') {
+              if (!existingRepo) {
+                // New repo - fetch README
+                shouldFetchReadme = true;
+              } else if (existingRepo.last_commit !== repoInfo.pushedAt) {
+                // Repo updated since last index - fetch README
+                shouldFetchReadme = true;
+              }
+              // else: repo unchanged, skip README fetch
+            }
+
+            if (shouldFetchReadme) {
               const repoReadme = await github.getReadme(repo.url);
               if (repoReadme) {
                 const textContent = extractTextContent(repoReadme.content);