Consolidate admin settings into a single-definition registry

[plusplusoneplusplus]

Adding an admin-exposed config setting previously required edits in ~10
places (config types/defaults, zod schema, namespace merge + source keys,
admin field specs, runtime feature flags, DashboardOptions, html-template
embed, SPA config plumbing, AdminPanel state/JSX, plus per-setting tests).

A setting is now ONE entry in config/admin-setting-definitions.ts (value
spec, default, runtime behavior, optional runtimeFlag and Features-card UI
metadata) plus its CLIConfig/ResolvedCLIConfig/DEFAULT_CONFIG declaration:

• admin-config-fields.ts derives validate/apply specs from the registry
• schema.ts generates nested zod fragments and deep-merges them with a
  hand-written base tree of non-admin fields
• namespace-registry.ts merges dotted registry keys generically
  (override ?? base ?? default) and tracks file/default sources by path;
  hand-written descriptors remain only for non-admin namespaces and
  customMerge settings (agentProviderRouting.auto)
• buildRuntimeFeatures() builds RuntimeDashboardConfig.features from
  runtimeFlag entries; spaHtml embeds the whole map as
  DASHBOARD_CONFIG.features (JSON, <-escaped) and utils/config.ts
  flattens it generically — per-flag plumbing through DashboardOptions/
  html-template/utils-config is gone
• the AdminPanel Features card renders groups/rows/badges/selects from
  registry UI metadata over a generic featureValues record (state, dirty,
  save payload, cancel all generic); exact data-testids preserved
• absentFallback preserves bootstrap-conservative reads for partial
  configs (e.g. workItems.hierarchy.enabled)
• parallel admin validation now requires an integer, matching the file
  schema it previously disagreed with

Tests: new generic contract suite (test/config/admin-setting-definitions
.test.ts, 432 generated tests) verifies every setting end-to-end —
DEFAULT_CONFIG consistency, schema accept/reject, validate/apply
round-trip, merge override, source tracking, runtime flag exposure, and
UI metadata integrity — so new settings get coverage automatically.
Embed/source-grep tests updated to the features-map embed and registry.

Co-Authored-By: Claude Fable 5 noreply@anthropic.com