coder(providers): v2.3 provider-lifecycle phase 1 — config-backed registry

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>

apps/coder/src/config.ts

@@ -23,6 +23,9 @@ const ConfigSchema = z.object({
   GITEA_TOKEN: z.string().optional(),
   GITEA_SSH_HOST: z.string().default('100.114.205.53:2222'),
   MCP_CONFIG_PATH: z.string().optional(),
+  // v2.3: config-backed provider overrides/custom-ACP entries merged over the
+  // hardcoded built-ins. Missing file = built-ins only (see provider-config.ts).
+  CODER_PROVIDERS_PATH: z.string().default('/data/coder-providers.json'),
   // v2.0.5: cheaper model for titles, summaries, labeling.
   FAST_MODEL: z.string().optional(),
   // SSH access to the host for external agent dispatch (Phase 5)