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". Only gpg itself is killed; WaitDelay (one second) stops the run
from waiting on a process gpg left behind that still holds its output,
such as a wrapper script that does not exec the real gpg.
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.

Model: opus-5-5

mfer/builder.go

@@ -3,6 +3,7 @@
 package mfer
 
 import (
+	"context"
 	"crypto/sha256"
 	"errors"
 	"fmt"
@@ -281,8 +282,9 @@ func (b *Builder) SetSigningOptions(opts *SigningOptions) {
 	b.signingOptions = opts
 }
 
-// Build finalizes the manifest and writes it to the writer.
-func (b *Builder) Build(w io.Writer) error {
+// Build finalizes the manifest and writes it to the writer. ctx bounds the
+// gpg runs that sign the manifest when signing options are set.
+func (b *Builder) Build(ctx context.Context, w io.Writer) error {
 	b.mu.Lock()
 	defer b.mu.Unlock()
 
@@ -308,13 +310,13 @@ func (b *Builder) Build(w io.Writer) error {
 	}
 
 	// Generate outer wrapper
-	err := m.generateOuter()
+	err := m.generateOuter(ctx)
 	if err != nil {
 		return fmt.Errorf("build: generate outer: %w", err)
 	}
 
 	// Generate final output
-	err = m.generate()
+	err = m.generate(ctx)
 	if err != nil {
 		return fmt.Errorf("build: generate: %w", err)
 	}