feat: Phase 8 — nomos packages extracted into internal/nomos/{turngate,retrycap,messagequeue,assent,session}

Mechanical extraction of nomos internal components into plain-Go subpackages
per the hexagonal plan (ADR 0016 §3.1 rule 3):
  turngate/   — per-session turn serialization (plan 2026-08-03 F1)
  retrycap/   — per-turn run retry cap (maxRunRetries=3)
  messagequeue/ — operator-message queue for busy-turn re-entry (F2)
  assent/     — chat-assent detection (isAssent, isTypedConfirmation,
                ExtractPendingApprovals), decoupled from agent via
                []string input instead of persistedCall
  session/    — store (chat sessions, plan execution, DB persistence),
                migration runner + local emitEvent to break adapter
                dependency

internal/migrate/ — shared migration runner extracted from postgres pool,
                    used by both the oikos postgres adapter and session tests.

session package export-rename finishing touches remain; the four smaller
packages compile with passing tests. Depguard rules and ADR-0016 leaf-note
update deferred to a followup. VERSION 0.35.1.

cmd/nomos/continue_test.go

@@ -54,7 +54,7 @@ func TestExtractExecutionIDs(t *testing.T) {
 // would dereference the nil provider and panic. Returning false cleanly proves
 // the body was skipped.
 func TestResumeSession_SkipsWhenBusy(t *testing.T) {
-	a := &agent{gate: newTurnGate()}
+	a := &agent{gate: turngate.New()}
 	if !a.gate.acquire("sess", 0) {
 		t.Fatal("precondition: initial acquire should succeed on a free session")
 	}
@@ -69,7 +69,7 @@ func TestResumeSession_SkipsWhenBusy(t *testing.T) {
 // next worker tick) instead of running or marking it. It must return cleanly
 // without reaching resumeSession's body (nil provider → panic) or markContinued.
 func TestContinueSession_DefersWhenBusy(t *testing.T) {
-	a := &agent{gate: newTurnGate()}
+	a := &agent{gate: turngate.New()}
 	if !a.gate.acquire("sess", 0) {
 		t.Fatal("precondition: initial acquire should succeed on a free session")
 	}