fix: adopt estate health envelope kit, fix blank BUILD_TIME

[vreshch]

Summary

Adopts @agentage/observability@0.8.0 (subpath /health) for landing's /health
route per ~/vaults/agentage/specs/health-endpoints.md (P2e). Fixes three live
defects visible in prod's current response:

• buildTime: "" → null. process.env.BUILD_TIME ?? null passed the empty
  string through; the kit's || null does not.
• commit was the full 40-char sha → now the 7-char short form (H12); the
  full sha now lives in version.
• version / startedAt / uptimeSeconds were missing entirely → now present.

service stays agentage-landing via the kit's OTEL_SERVICE_NAME default
(already set to agentage-landing in docker-compose.yml for tracing) - no
rename needed in the route itself, no service argument passed.

Root cause of the blank BUILD_TIME

deploy.yml's image build only ever passed COMMIT_SHA as a build-arg. The
Dockerfile's ARG BUILD_TIME="" default was therefore baked into every image,
which is why prod has shipped buildTime: "" since day one. Fixed by generating
a UTC timestamp in a vars step and passing it as BUILD_TIME, matching the
pattern already used in auth/dashboard/admin/api-gateway/web.

Changes

• packages/landing/package.json - bump @agentage/observability ^0.6.1 -> ^0.8.0
  (already a dependency, used for OTEL bootstrap via the /next subpath)
• packages/landing/src/app/health/route.ts - replace the hand-rolled response
  with healthResponse() from @agentage/observability/health
• packages/landing/src/app/health/route.test.ts - new unit test asserting the
  v1 envelope shape (7-char commit, null buildTime on a blank env var, service
  name, version/startedAt/uptimeSeconds)
• .github/workflows/deploy.yml - generate + pass BUILD_TIME as a build-arg

No dependency checks added, per the spec's Next.js profile (SSR shell, no
downstream probes). Dockerfile HEALTHCHECK unchanged, verified still passes.

Test plan

• npm run verify green (type-check, lint, format, unit tests, real next build)
  - /health shows as ƒ (Dynamic) in the build output, confirming no prerender
• docker build --target runner --build-arg COMMIT_SHA=testsha1234567890 (blank
  BUILD_TIME) -> ran container -> curl /health -> buildTime: null,
  commit: "testsha", version: "testsha1234567890"
• Re-ran with OTEL_SERVICE_NAME=agentage-landing (matches prod compose) ->
  service: "agentage-landing"
• Re-built with BUILD_TIME set -> got a real ISO 8601 UTC timestamp, confirming
  the deploy.yml fix produces the right value
• Dockerfile HEALTHCHECK (wget -q --spider http://localhost:3000/health)
  still reports healthy

Not deployed - draft only, per repo convention (deploy.yml ships both dev+prod on
push to master).

[github-actions]

✅ CI - Verify PASSED

Commit: 8c33e03871093a5c36cec9de90f7445a16aef3ce · Branch: feature/health-kit

• ✅ Type check
• ✅ Lint
• ✅ Format check
• ✅ Unit tests
• ✅ Build
• ✅ Landing e2e

🔗 View workflow run