feat: Phase 0 of hexagonal refactor — ADR 0016, core scaffold, depguard rules

Problem: the hexagonal-architecture plan (plans/2026-08-15-hexagonal-
architecture.md) needs its foundation — an accepted ADR, the target
directory tree, and machine-checked dependency rules — before any
service extraction starts. Also folds the four outstanding review
findings (F3.1/F5/F6/F7) into the plan: ObservationService owns the
bounded probe-concurrency contract (scheduler.go:133), Phase 9 gates
ExecutionService+PolicyService ≥ 90% with a gating-matrix test,
per-phase abort criteria, and the §3.2 internal/config note.

Change:
- docs/adr/0016-hexagonal-ports-adapters.md records context, decision,
  and consequences of the ports & adapters migration.
- internal/domain → internal/core/domain (mechanical import rewrite,
  20 files), new internal/core/{ports,app}, internal/adapters trees
  with package docs.
- .golangci.yml: depguard rules for §3.1 (core purity, no agent-client
  tech in core, nomos isolation — the nomos rules self-activate when
  internal/nomos exists in Phase 8). Config migrated to golangci-lint
  v2 format so it loads at all (the v1 config errored under v2, masked
  by CI's advisory continue-on-error). Verified depguard fires on a
  planted openai-go import in internal/core/app.
- CONTRIBUTING.md layout section now shows the core/adapters tree.

Risk: import path churn is mechanical and tests pass unchanged; the
lint config migration surfaces the pre-existing 400-issue baseline
(advisory in CI, unchanged policy) — new/moved packages lint clean.

Verification: go vet ./..., make test (race, core/domain at 100%
coverage), make generate-check, golangci-lint on internal/core/... and
internal/adapters/... — 0 issues; depguard violation probe confirmed.

plans/index.md

@@ -22,6 +22,7 @@ went sideways, open an investigation.
 | 2026-08-04 | [Hermes MCP client integration](done/2026-08-04-hermes-mcp-client-integration.md) | Done — deployed |
 | 2026-08-05 | [Agent execution safety: QEMU guest agent gate + host-mutation guard](done/2026-08-05-agent-execution-safety-qemu-guest-agent-gate.md) | Done — implemented (1b9c761) |
 | 2026-08-05 | [Backend evaluation: architecture, security, and reliability improvements](2026-08-05-backend-evaluation-improvements.md) | Done — all three phases (B, D, E) implemented as code (0.28.0–0.29.0), deployed, and hardened via review. Remaining: C (security) and F (performance) backlog. |
+| 2026-08-15 | [Hexagonal architecture — design and phased refactor](2026-08-15-hexagonal-architecture.md) | In Progress — Phase 0 done (ADR 0016, core scaffold, domain moved, depguard) |
 
 ## Done