refactor: use SDK for loading local agents with async initialization

- Add initializeAgentRegistry() async function called at CLI startup
- Replace sync require-based agent loading with SDK loadLocalAgents()
- Remove dead code: gatherAgentFiles, regex patterns, SKIPPED_DIRECTORIES
- User agents now cached after SDK loads them via esbuild transpilation
- Preserve file paths for user agents (for UI "Open file" links)
- Update tests to work with bundled agents present (use test- prefix)
- Add 24 new tests for loadLocalAgents, handleSteps, utilities, edge cases

Author: brandonkachen

.env.example

@@ -22,6 +22,10 @@ STRIPE_TEAM_FEE_PRICE_ID=price_dummy_team_fee_id
 LINKUP_API_KEY=dummy_linkup_key
 LOOPS_API_KEY=dummy_loops_key
 
+# Admin/Internal Tools
+# Required for admin relabeling operations (POST /api/admin/relabel-for-user)
+CODEBUFF_API_KEY=dummy_codebuff_api_key
+
 # Discord Integration
 DISCORD_PUBLIC_KEY=dummy_discord_public_key
 DISCORD_BOT_TOKEN=dummy_discord_bot_token