fix: enrollment FK race + log-when-rejected; runbook fixes from dry-run

The smoke runbook caught a real bug: ConsumeEnrollmentToken was
inserting into host_credentials (FK -> hosts) inside the same tx as
the token burn, but the host row didn't exist yet — CreateHost
runs in the *next* statement. The agent saw a generic 401 with no
clue why.

Fix: drop the host_credentials insert from ConsumeEnrollmentToken;
the HTTP handler now does Consume -> CreateHost ->
SetHostCredentials. SetHostCredentials failure is logged loudly
but doesn't fail the enrol — operator recovers via PUT
/api/hosts/{id}/repo-credentials.

Adds slog.Warn lines on both 401 paths in handleAgentEnroll so the
underlying cause is visible in server logs (the wire response stays
generic to avoid leaking which step failed).

Test: TestEnrollmentTransfersRepoCreds rewritten to mirror the new
order (consume -> create host -> SetHostCredentials).

Runbook (docs/e2e-smoke.md): rest-server moved off 8000 (commonly
in use); URLs use trailing slash on the rest path; clarified that
secrets_key is minted on first agent start, not at enrol time.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/server/http/host_credentials_test.go

@@ -46,14 +46,18 @@ func TestEnrollmentTransfersRepoCreds(t *testing.T) {
 		t.Errorf("rebind should change ciphertext (additional-data differs); got identical")
 	}
 
-	// Need a host row for the FK.
+	// Burn the token, then create the host row, then promote — same
+	// order the HTTP handler runs.
+	if err := st.ConsumeEnrollmentToken(ctx, tokHash, hostID); err != nil {
+		t.Fatalf("consume: %v", err)
+	}
 	if _, err := st.DB().Exec(
 		`INSERT INTO hosts (id, name, os, arch, enrolled_at) VALUES (?,?,?,?,?)`,
 		hostID, "host42", "linux", "amd64", "2026-01-01T00:00:00Z"); err != nil {
 		t.Fatalf("insert host: %v", err)
 	}
-	if err := st.ConsumeEnrollmentToken(ctx, tokHash, hostID, encForHost); err != nil {
-		t.Fatalf("consume: %v", err)
+	if err := st.SetHostCredentials(ctx, hostID, encForHost); err != nil {
+		t.Fatalf("set host credentials: %v", err)
 	}
 
 	// host_credentials row should now hold the host-bound ciphertext.