ah-nyl.6 wiring: httpapi + agenthubd + ahub CLIs Past Stand

Wire the daemon together per docs/SPEC.md SS3, SS11 (read SPEC fully). All sibling packages now exist — full-tree builds are allowed and expected in THIS task.

Deliverables:
- internal/httpapi: loopback server per SS11 — GET /healthz; POST /internal/v1/run-exit {task_id,attempt,exit_code} validated -> calls a RunExitHook (func injected by main; it pokes the reconciler); GET /api/v1/status -> JSON {tasks:[TaskRecord+latest Run], generated_at}. stdlib net/http + 1.22 mux patterns; graceful shutdown; tests via httptest.
- cmd/agenthubd: flags --config (default ./agenthub.yaml) --log-level; load config, open store, construct vikunja client, runner, no-op notifier (slog), reconciler; start httpapi + reconciler.Run; SIGINT/SIGTERM -> graceful stop (context cancel, http shutdown, store close). slog JSON to stderr.
- cmd/ahub: subcommands (stdlib flag, no cobra): `status` (GET /api/v1/status from --addr default 127.0.0.1:9100, human-readable table + --json raw), `validate-config` (load config, print OK or the collected errors, exit code accordingly), `version` (var set via -ldflags, default "dev").
- justfile: verify `just build` produces bin/agenthubd bin/ahub bin/ahub-run (adjust if needed).
- Smoke check you must run and make pass: `go build ./... && go vet ./... && go test ./...` (whole tree), then `bin/agenthubd --config config.example.yaml` with a fake VIKUNJA_TOKEN env — it must start, log the bucket-resolution failure gracefully (retry next tick, not crash-loop-exit), and /healthz must answer. Note in the bead comment if SPEC/behavior forced any deviation.

Constraints: you own cmd/agenthubd, cmd/ahub, internal/httpapi, plus minimal glue edits elsewhere ONLY if a sibling package has an integration bug you must fix to link (document any such fix in its own commit '<pkg>: fix ...'). No .beads/ changes. Commits: 'httpapi: ...', 'cmd: ...'. Do NOT push.

CONVENTION UPDATE (see SPEC §13, commit 5ce35c5+): tests use testify (require/assert); errors via auxilia culpa; agenthubd wires scribe handlers (TintHandler for ahub CLI, JSON or Multi for the daemon); steward MAY be used for daemon lifecycle wiring if it stays simple. testify + auxilia already in go.mod.
REVIEW INPUT (data-layer review finding 4): agenthubd startup must PREFLIGHT the board — construct the vikunja client and resolve the configured bucket titles once at startup; a missing title / no-kanban-view error at that point is FATAL with a clear message (SPEC §8 'fatal config error'), while the same error later at runtime stays retryable inside the loop. Wire this into cmd/agenthubd.