Adds a config layer merged over the hardcoded built-ins (tasks 1.1-1.6): CODER_PROVIDERS_PATH env (default /data/coder-providers.json); provider-config.ts (Zod schema + never-throw loader — missing/invalid file falls back to built-ins only — + save); provider-config-registry.ts (ResolvedProviderDef + buildResolvedRegistry merge: override built-ins, add custom extends:'acp' entries, boocode always enabled + singleton); agent-probe now iterates the resolved registry, probes custom-ACP command[0] via execFile (no shell), skips disabled providers (keeps the row), reads enabled from memory only (no DB column). No snapshot/dispatch/route/UI changes (Phase 2+). 6 new unit tests; empty config provably yields exactly the built-ins.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>