feat: Phase 2 — ports package, secrets port move, postgres adapter move

Problem: the hexagon's Phase 2 (plans/2026-08-15-hexagonal-architecture.md)
must give the use-cases-to-be their contract surface: driven-port
interfaces, test fakes, the secrets interface moved into core, and the
postgres package inside the adapters tree — before the first vertical
slice (Phase 3) can wire a composition root.

Change:
- internal/core/ports: full driven-port catalog per plan §3.3 —
  repositories as transaction-scoped aggregates whose inputs carry
  derived checks, audit, and events (§3.6), plus CommandExecutor,
  TargetResolver, Checker, Secrets, EventPublisher, Provisioner.
  Port-local payload types (Event, AuditEntry, CheckDef, KnowledgeEntry,
  ExecResult) keep signatures off infrastructure; TypeTree aliases
  internal/ontology (pure over domain) until checkdefaults is absorbed.
  ReadModels intentionally not declared yet — it materializes with the
  Phase 3 slice and grows as report handlers rewire.
- secrets.Backend is now an alias of ports.Secrets; implementations
  (Infisical, SOPS, Manager) unchanged. mcp's local secretBackend
  subset is deleted; tool constructors take ports.Secrets.
- internal/db → internal/adapters/postgres (mechanical import rewrite;
  package identifier stays db until the Phase 3 repository split).
  sqlc.yaml, Makefile, golangci exclusions, and docs follow the move;
  make generate-check verified.
- internal/adapters/ssh: Executor implements ports.CommandExecutor over
  the actuator dial pool + RunStreaming (10-min default timeout carried
  over from the httpapi path).
- internal/adapters/remote: Resolver implements ports.TargetResolver
  delegating to internal/remote (still pool-based; drops onto
  ports.EntityRepository when repositories land in Phase 3 — documented
  transitional import).
- internal/core/ports/portstest: importable fakes — in-memory
  EntityRepo (with check-then-act SetState, side-effect recording),
  RecordingExecutor, FakeChecker, SpyPublisher; port-satisfaction
  guards; tests.

Risk: ports are declared ahead of implementations — signatures firm up
per phase as slices land (documented in the package doc); the
remote→postgres transitional import is explicit and dissolves in
Phase 3.

Verification: go vet, make test (race, 19 packages), generate-check,
golangci on core+adapters — 0 issues; full-repo baseline down
365→344.

internal/core/ports/observation.go

@@ -0,0 +1,61 @@
+package ports
+
+import (
+	"context"
+	"encoding/json"
+	"time"
+
+	"github.com/dtoro/oikos/internal/core/domain"
+)
+
+// CheckDef is a health-check definition (subset of the check_defs row the
+// observe pass consumes; grows with the probe adapters in Phase 5).
+type CheckDef struct {
+	ID         domain.UUID
+	EntityID   domain.UUID
+	Kind       string
+	Name       string
+	Config     json.RawMessage
+	IntervalS  int
+	Enabled    bool
+	Severity   string
+	CreatedAt  time.Time
+	UpdatedAt  time.Time
+}
+
+// CheckResult is what one probe run produces.
+type CheckResult struct {
+	Value   float64
+	State   string // "ok", "warning", "critical", "unknown"
+	Message string
+}
+
+// Checker probes one check kind. One adapter per kind under adapters/probes;
+// the ObservationService picks the adapter by CheckDef.Kind.
+type Checker interface {
+	Check(ctx context.Context, def CheckDef, target Target) CheckResult
+}
+
+// MetricSample is one metric observation to record.
+type MetricSample struct {
+	Metric    string
+	Value     float64
+	Timestamp time.Time
+}
+
+// MetricsRepository records observe-pass metrics (Timescale write path;
+// bucketed/trend reads belong to ReadModels).
+type MetricsRepository interface {
+	InsertSamples(ctx context.Context, entityID domain.UUID, samples []MetricSample) error
+}
+
+// CheckRepository — monitoring definitions. EnsureFor is the read-diff-write
+// derivation in one transaction.
+type CheckRepository interface {
+	ListEnabled(ctx context.Context) ([]CheckDef, error)
+	ListFor(ctx context.Context, entityID domain.UUID) ([]CheckDef, error)
+	// EnsureFor inserts defs that are missing, updates changed ones, and
+	// removes stale ones for the entity — atomically.
+	EnsureFor(ctx context.Context, entityID domain.UUID, desired []CheckDef) error
+	SetEnabled(ctx context.Context, checkID domain.UUID, enabled bool) error
+}