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/secrets/backend.go

@@ -6,18 +6,17 @@ import (
 	"log/slog"
 	"sync"
 	"time"
+
+	"github.com/dtoro/oikos/internal/core/ports"
 )
 
 var ErrNotFound = errors.New("secret not found")
 var ErrBackendUnavailable = errors.New("secret backend unavailable")
 
-// Backend is the interface for retrieving and storing secrets.
-type Backend interface {
-	Get(ctx context.Context, key string) (string, error)
-	List(ctx context.Context) ([]string, error)
-	Set(ctx context.Context, key string, value string) error
-	Name() string
-}
+// Backend is the secrets port. The interface lives in core/ports (ADR 0016);
+// this alias keeps existing call sites working while implementations
+// (Infisical, SOPS, Manager) stay in this package.
+type Backend = ports.Secrets
 
 // Manager holds a primary and fallback backend. If the primary fails,
 // it falls back to the secondary. Supports periodic background refresh