Enforce real timeouts on gpg subprocess calls (closes #62)

Every gpg run now has a one-minute deadline (gpgTimeout) on top of its
caller's context and is killed when either ends. A timeout is reported
as "gpg timed out" under the failing operation instead of "signal:
killed". Builder.Build and Checker.ExtractEmbeddedSigningKeyFP take a
context, so ToManifest's context now reaches signing and the
contextcheck suppression calling signing non-cancellable is gone.
Manifest loading takes no context, so its signature check is bounded by
the timeout alone. Killing gpg itself is enough: the gpg-agent it starts
runs detached and holds none of its output.

Model: opus-5-5

internal/cli/check.go

@@ -2,6 +2,7 @@
 package cli
 
 import (
+	"context"
 	"encoding/hex"
 	"errors"
 	"fmt"
@@ -126,7 +127,9 @@ func (mfa *CLIApp) fetchManifestToTemp(url string) (string, error) {
 
 // verifyRequiredSigner enforces the --require-signature fingerprint
 // against the manifest's embedded signing key.
-func verifyRequiredSigner(chk *mfer.Checker, requiredSigner string) error {
+func verifyRequiredSigner(
+	ctx context.Context, chk *mfer.Checker, requiredSigner string,
+) error {
 	// Validate fingerprint format: must be exactly 40 hex characters
 	if len(requiredSigner) != fingerprintHexLen {
 		return fmt.Errorf("%w, got %d", errInvalidFingerprint, len(requiredSigner))
@@ -145,7 +148,7 @@ func verifyRequiredSigner(chk *mfer.Checker, requiredSigner string) error {
 	// Extract fingerprint from the embedded public key (not from the
 	// signer field). This validates the key is importable and gets its
 	// actual fingerprint.
-	embeddedFP, err := chk.ExtractEmbeddedSigningKeyFP()
+	embeddedFP, err := chk.ExtractEmbeddedSigningKeyFP(ctx)
 	if err != nil {
 		return fmt.Errorf(
 			"failed to extract fingerprint from embedded signing key: %w", err)
@@ -303,7 +306,7 @@ func (mfa *CLIApp) checkManifestOperation(ctx *cli.Context) error {
 	// Check signature requirement
 	requiredSigner := ctx.String("require-signature")
 	if requiredSigner != "" {
-		err = verifyRequiredSigner(chk, requiredSigner)
+		err = verifyRequiredSigner(ctx.Context, chk, requiredSigner)
 		if err != nil {
 			return err
 		}