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.

CONTRIBUTING.md

@@ -60,7 +60,9 @@ internal/           All Go packages
                     here phase by phase
   httpapi/          REST + MCP server (OpenAPI-generated)
   mcp/              MCP tool implementations
-  db/               Connection pool, migrations, seeds, sqlc queries
+  db/               (moved) → internal/adapters/postgres: pool, migrations,
+                     seeds, sqlc queries — package still named `db` until
+                     the Phase 3 repository split
   scheduler/        Observe loop, probes, signals
   actuator/         SSH execution
   learning/         Pattern recognition, anomaly detection
@@ -116,8 +118,8 @@ Never hand-edit `internal/httpapi/gen/api.gen.go`.
 
 ### Database access is sqlc-first
 
-SQL queries live in `internal/db/queries/*.sql`. Go code is generated with
-`sqlc` into `internal/db/sqlcgen/`. Config in `sqlc.yaml`.
+SQL queries live in `internal/adapters/postgres/queries/*.sql`. Go code is generated with
+`sqlc` into `internal/adapters/postgres/sqlcgen/`. Config in `sqlc.yaml`.
 
 - Queries target pgx/v5 with UUID + timestamptz overrides
 - Never hand-edit generated sqlc code