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/approval.go

@@ -0,0 +1,110 @@
+package domain
+
+import "time"
+
+// Approval is a short-TTL signed grant for a gated action.
+type Approval struct {
+	EntityID         UUID
+	SubjectEntityID  UUID
+	Action           string
+	RiskClass        string
+	Kind             string
+	Payload          map[string]any
+	Status           string
+	TokenHash        string
+	ExpiresAt        time.Time
+	DecidedAt        *time.Time
+	DecidedBy        UUID
+	CreatedAt        time.Time
+}
+
+// Approval statuses.
+const (
+	ApprovalPending  = "pending"
+	ApprovalApproved = "approved"
+	ApprovalDenied   = "denied"
+	ApprovalExpired  = "expired"
+	ApprovalRevoked  = "revoked"
+)
+
+// Approval kinds.
+const (
+	ApprovalKindExecution        = "execution"
+	ApprovalKindPolicyChange     = "policy-change"
+	ApprovalKindPatternActivation = "pattern-activation"
+)
+
+// CheckDef defines a probe (R3-7: probes as data, not code).
+type CheckDef struct {
+	EntityID   UUID
+	TargetID   UUID
+	TargetType string
+	Kind       string
+	Config     map[string]any
+	IntervalS  int
+	TimeoutS   int
+	Zone       string
+	Enabled    bool
+	UpdatedAt  time.Time
+}
+
+// Check kinds.
+const (
+	CheckHTTP      = "http"
+	CheckTCP       = "tcp"
+	CheckDisk      = "disk"
+	CheckCertExpiry = "cert-expiry"
+	CheckDrift     = "drift"
+	CheckSSHScript = "ssh-script"
+)
+
+// EntityStatus is the current health of an entity (R3-6: replaces state_snapshots).
+type EntityStatus struct {
+	EntityID    UUID
+	Health      string
+	LastCheckAt *time.Time
+	Details     map[string]any
+	UpdatedAt   time.Time
+}
+
+// Health values.
+const (
+	HealthHealthy  = "healthy"
+	HealthDegraded = "degraded"
+	HealthDown     = "down"
+	HealthUnknown  = "unknown"
+)
+
+// RiskClass is the four-level safety model.
+type RiskClass struct {
+	Name              string
+	Description       string
+	ApprovalRequired  string
+	AutonomyAllowed   bool
+}
+
+// Risk class names.
+const (
+	RiskReadOnly       = "read_only"
+	RiskReversibleLow  = "reversible_low"
+	RiskConfigMutation = "config_mutation"
+	RiskDestructive    = "destructive"
+)
+
+// ApprovalRule maps (entity_type, action) → risk_class + autonomy.
+type ApprovalRule struct {
+	ID           UUID
+	EntityType   string
+	Action       string
+	RiskClass    string
+	AutonomyLevel string
+	ScopeEntity  UUID
+	Version      int
+}
+
+// Autonomy levels.
+const (
+	AutonomyAuto     = "auto"
+	AutonomyEscalate = "escalate"
+	AutonomyNever    = "never"
+)