Load backend smoke-test metadata

docs/formats.md

@@ -168,16 +168,46 @@ are not reserved against configuration changes.
 | `rakestrawhome` | `https://inference.ai.rakestrawhome.com/v1` | `RAKESTRAWHOME_INFERENCE_API_KEY` | 4 |
 
 A backend definition may have `endpoint`, `api_key_env`,
-`concurrency_limit`, `queue_capacity`, and `extra_params`. The final endpoint
-is required and must be an absolute HTTP(S) URL with a host, no user
-information, query, or fragment. An omitted or null API-key environment name
-means no lookup. Omitted/null concurrency is zero (unlimited); a supplied
+`concurrency_limit`, `queue_capacity`, `extra_params`, and `smoke_tests`. The
+final endpoint is required and must be an absolute HTTP(S) URL with a host, no
+user information, query, or fragment. An omitted or null API-key environment
+name means no lookup. Omitted/null concurrency is zero (unlimited); a supplied
 negative value is invalid. For positive concurrency, omitted/null queue
 capacity defaults to 1024. An explicit queue capacity, including zero,
 requires positive concurrency and must be nonnegative. Unlimited concurrency
 has no queue. Provider maps contain JSON-compatible values; null entries are
 ordinary JSON null. Unknown fields, including a raw `api_key`, fail.
 
+`smoke_tests` is optional backend metadata for explicit diagnostic operations.
+It may contain `default_profile`, `reachability`, and `authentication`:
+
+```yaml
+backends:
+  openrouter:
+    smoke_tests:
+      default_profile: promptkit-openrouter-standard
+      reachability:
+        endpoint: https://status.example/ready?check=1
+      authentication:
+        kind: openrouter_key
+        endpoint: https://openrouter.ai/api/v1/key
+```
+
+An omitted or null `smoke_tests` block disables these metadata defaults.
+`default_profile` is preserved as an exact, nonblank profile ID; its existence
+is checked only if a diagnostic selects that default. Omitted/null
+`reachability` or its endpoint uses the effective backend endpoint. An explicit
+reachability endpoint is trimmed and must be an absolute HTTP(S) URL with a
+host; unlike the backend base endpoint, its path and query are allowed. Null
+means fallback, while an explicit blank value is invalid.
+
+Omitted/null `authentication` means no authentication adapter is declared. A
+non-null object requires `kind`, which is trimmed and case-normalized; the
+recognized values are `none` and `openrouter_key`. Its omitted/null endpoint
+uses the adapter's backend-relative destination; an explicit endpoint follows
+the same full-URL rules as reachability. These values are configuration only:
+engine construction does not contact a backend.
+
 ## Profile Definitions
 
 Profiles are keyed by literal ID and may inherit a named parent with

docs/roadmap/implementation.md

@@ -264,6 +264,10 @@ Validate: `go test ./internal/configuration ./internal/backend` and focused root
 backend-source tests. Update backend-field GoDoc and `docs/formats.md` as these
 configuration fields land; runtime smoke-test guidance waits for Stage 18.
 
+**Completed: 2026-10-13.** Validation passed:
+`go test ./internal/configuration ./internal/backend`, `go test .`, and
+`git diff --check`.
+
 ## Stage 3: Resolve Diagnostic Targets Without Preparing Applications
 
 Scope: selection helpers in `internal/usecase`; no transport.