docs(development): add observability dev-loop section

ADR-0012 phase 1 + phase 2 are wired (BFF + SPA), so the
"Observability dev-loop" placeholder in the roadmap table now has
real content to point at. Promote it from §9 (was §8) future-work
list to a full §5 walkthrough sitting between "Daily commands"
and "Dependency updates".

What §5 covers:

- Bringing up the observability stack (`./infra/local/dev.sh up
  observability`) with the endpoint table.
- Reading a trace in Jaeger — service-dropdown filter, span
  attribute keys to look at, the trace-id-as-pivot pattern.
- Correlating a trace with the BFF Pino logs via `trace_id` and
  `request_id` (the latter the per-request UUID nestjs-cls
  provides).
- Reading SPA spans from the browser (DevTools Network +
  Jaeger UI cross-check).
- Common gotchas — observability profile not active, CORS
  pre-flight on `/v1/traces`, `propagateTraceHeaderCorsUrls`
  mismatches, NODE_ENV mis-set, EADDRINUSE.
- Pointer at the "wired as features land" deltas (CLS keys for
  session/user/audience, redact list, custom domain spans, prod
  backend) so a contributor knows what's intentionally not yet
  there.

Renumber the rest of the doc accordingly (5→6 / 6→7 / 7→8 / 8→9)
and drop the now-implemented "Observability dev-loop" line from
the §9 roadmap table.

`docs/README.md` cross-link updated to mention the new section.

docs/README.md

@@ -13,7 +13,7 @@ This is the entry point to all project documentation. It is maintained automatic
 
 ### Daily development
 
-- [development.md](development.md) — repo layout, prerequisites, initial setup, daily commands, dependency updates (Renovate), conventional commit cycle. Day-to-day reference for working on the project.
+- [development.md](development.md) — repo layout, prerequisites, initial setup, daily commands, observability dev-loop (Jaeger UI, log ↔ trace correlation), dependency updates (Renovate), conventional commit cycle. Day-to-day reference for working on the project.
 
 ### Architecture