✨ feat(server): wire sampler hot-reload callback (ADR-0023 Phase 3, sub-phase 3.3)

Closes ADR-0023 hot-reload Phase 3 (telemetry sampler). Last sub-phase
of the 3-PR sequence: 3.1 = ReconfigureTracerProvider in pkg/telemetry
(PR #45), 3.2 = sampler callback plumbing in pkg/config (in main via
PR #47 race-merge — see Q-038 in mistral-quirks.md), 3.3 = wire it in
pkg/server.Run (this PR).

Implementation:
- Capture telemetrySetup pointer (was a local) so it outlives initial
  setup and the sampler callback can mutate it.
- After WatchAndApply registration, register a callback that:
  1. Mutates SamplerType/SamplerRatio on the captured Setup
  2. Calls ReconfigureTracerProvider (which builds the new TP, swaps
     the global, drains the old)
  3. Updates s.tracerProvider so graceful shutdown still drains the
     correct provider.
- Callback only registered if telemetrySetup != nil (telemetry was
  successfully initialized at startup; hot-reloading telemetry-on is
  out of scope per ADR-0023).

ADR-0023 status updated: Phase 1+2+3 Implemented. api.v2_enabled
remains explicitly deferred (router refactor needed; separate ADR).

Race detector clean on touched packages. Full BDD suite green.

adr/0023-config-hot-reloading.md

@@ -1,6 +1,6 @@
 # Config Hot Reloading Strategy
 
-**Status:** Phase 1+2 Implemented (2026-05-05 — `logging.level` and `auth.jwt.ttl` hot-reloadable via `Config.WatchAndApply` in `pkg/config/config.go`, wired in `pkg/server/server.go Run`. Phase 2 also fixed a pre-existing bug where the hardcoded 24h TTL ignored `auth.jwt.ttl` from config entirely.) Phase 3 sub-phase 3.1 Implemented (2026-05-05 — `ReconfigureTracerProvider` in `pkg/telemetry/telemetry.go` added). Phase 3 sub-phase 3.2 In Flight (2026-05-05 — `telemetry.sampler.type` + `telemetry.sampler.ratio` hot-reload via `SetSamplerReconfigureCallback` in `pkg/config/config.go`. Remaining field: `api.v2_enabled`.)
+**Status:** Phase 1+2+3 Implemented (2026-05-05). Hot-reloadable fields: `logging.level`, `auth.jwt.ttl`, `telemetry.sampler.type`, `telemetry.sampler.ratio`. Plumbing: `Config.WatchAndApply` in `pkg/config/config.go`, `ReconfigureTracerProvider` in `pkg/telemetry/telemetry.go`, sampler reconfigure callback wired in `pkg/server/server.go Run`. Phase 2 also fixed a pre-existing bug where the hardcoded 24h TTL ignored `auth.jwt.ttl` from config. Remaining field `api.v2_enabled` is **deferred**: hot-reloading routing requires either an always-register-with-middleware-gate refactor of the chi router or an atomic router swap — different complexity class, separate ADR if reopened.
 **Authors:** Gabriel Radureau, AI Agent
 **Date:** 2026-04-05
 **Last Updated:** 2026-05-05