Address Gadfly findings on #90 (blocking round)

- Remove config.AgentConfig.Ready() — dead after the refactor (no non-test
  callers), and its doc comment ("route registration and capabilities gate on
  this") was now false. EffectiveAgent.Ready() carries the same logic and IS
  used, see below.
- agentHolder now uses eff.Ready() and eff.APIKey instead of an inline check and
  a redundant apiKey field it held separately. This makes EffectiveAgent.APIKey/
  Ready() production-used rather than test-only, drops the duplicated readiness
  check, and simplifies newAgentHolder's signature.
- settingsPayload no longer swallows an EffectiveAgent error into a misleading
  empty "effective" view (which would read as "nothing configured"). It returns
  the error; the handlers surface it as a 500. EffectiveAgent re-reads the row
  GetInstanceSettings just returned, so a failure there is a real DB fault.
- Frontend streamChat handles 503 (assistant disabled at runtime) distinctly
  from 404 (endpoint absent) — the backend returns 503 AGENT_DISABLED now, which
  the old code mislabelled.
- Fixed the api.go comment that still said a disabled request gets a 404 — it's
  a 503.
- updateSettings uses the shared parseNullable[bool] instead of a bespoke
  parseNullableBool (now removed).
- NewRunner drops its empty-spec guard; agentmodel.Resolve already owns that
  check, so one place decides what a valid spec is.
- rebuild-on-resolve-error now documents WHY it keeps the current Runner rather
  than tearing down a working assistant on a transient DB blip: the state is
  persisted, the next rebuild reconciles, and killing a live assistant on a read
  hiccup is worse than a brief stale window.

Not changed: the store's version-conflict fallback returning GetInstanceSettings'
error instead of ErrVersionConflict when that read also fails. That's the exact
pattern every other version-guarded update uses (gardens/objects/plants); making
only this one differ would be the inconsistency. A DB read failing immediately
after the guarded UPDATE on the same local SQLite file is a disk-fault edge case,
and surfacing that error is defensible.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

internal/agent/runtime.go

@@ -48,9 +48,11 @@ type Runner struct {
 // constructor serves both boot (from env) and a runtime settings change (from
 // the DB) — the Runner has no idea which one configured it.
 func NewRunner(svc *service.Service, apiKey, modelSpec string) (*Runner, error) {
-	if apiKey == "" || strings.TrimSpace(modelSpec) == "" {
+	if apiKey == "" {
 		return nil, errors.New("agent: not configured")
 	}
+	// agentmodel.Resolve already rejects an empty/blank spec, so don't duplicate
+	// that guard here — one place decides what a valid spec is.
 	model, err := agentmodel.Resolve(apiKey, modelSpec)
 	if err != nil {
 		return nil, err