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.

internal/core/domain/execution.go

@@ -0,0 +1,64 @@
+package domain
+
+import "time"
+
+// Classification persists every autonomous decision the classifier makes (SA5).
+// This is the audit trail for "why did the OS auto-act / escalate?"
+type Classification struct {
+	EntityID         UUID
+	SignalEntityID   UUID
+	TargetEntityID   UUID
+	Action           string
+	RecommendedAction map[string]any
+	RiskClass        string
+	Route            string
+	BlastRadius      []UUID
+	PatternConfidence float64
+	SkillID          UUID
+	AutonomyCheck    string
+	Reasoning        map[string]any
+	CorrelationID    string
+	CreatedAt        time.Time
+}
+
+// Classification routes.
+const (
+	RouteAutoAct   = "auto-act"
+	RouteEscalate  = "escalate"
+	RouteHold      = "hold"
+)
+
+// Execution is a detailed record of one action the OS performed.
+type Execution struct {
+	EntityID         UUID
+	ClassificationID UUID
+	SignalEntityID   UUID
+	TargetEntityID   UUID
+	Action           string
+	RiskClass        string
+	ApprovalID       UUID
+	AgentID          UUID
+	SkillID          UUID
+	SkillVersion     int
+	Status           string
+	Result           map[string]any
+	DurationMs       int
+	Verified         bool
+	CorrelationID    string
+	StartedAt        *time.Time
+	CompletedAt      *time.Time
+	CreatedAt        time.Time
+}
+
+// Execution lifecycle states.
+const (
+	ExecProposed    = "proposed"
+	ExecApproved    = "approved"
+	ExecExecuting   = "executing"
+	ExecVerified    = "verified"
+	ExecFailed      = "failed"
+	ExecTimedOut    = "timed-out"
+	ExecRolledBack  = "rolled-back"
+	ExecCancelled   = "cancelled"
+	ExecExpired     = "expired"
+)