Extract workspace subpackage with pure repo helpers

Moves the stateless parts of the workspace subsystem into
internal/daemon/workspace:

- RepoSpec struct + InspectRepo for host-side git inspection
- ImportRepoToGuest (taking a minimal GuestClient interface) with the
  full-copy and metadata-only / shallow-overlay paths
- FinalizeScript, PrepareRepoCopy, ResolveSourcePath
- ListSubmodules, ListOverlayPaths, ParsePrepareMode
- Git helpers (GitOutput, GitTrimmedOutput, GitResolvedConfigValue,
  ParseNullSeparatedOutput, RunHostCommand, GitFileURL) and the
  HostCommandOutputFunc test seam
- ShallowFetchDepth const

The subpackage imports internal/daemon/session for ShellQuote and
FormatStepError so both workspace and session pure helpers live in
their own subpackages with a clean session→workspace direction of use.

daemon/workspace.go shrinks from 481 → 156 LOC, keeping just the three
orchestrator methods (Export, Prepare, prepareLocked) that still touch
d.store, d.FindVM, d.dialGuest, d.waitForGuestSSH, and the VM lock set.
guestSessionHostCommandOutputFunc is removed from guest_sessions.go (its
only caller was workspace.go; the new package has its own copy).

All tests green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/daemon/guest_sessions.go

@@ -18,20 +18,6 @@ import (
 	"banger/internal/system"
 )
 
-var guestSessionHostCommandOutputFunc = func(ctx context.Context, name string, args ...string) ([]byte, error) {
-	runner := system.NewRunner()
-	output, err := runner.Run(ctx, name, args...)
-	if err == nil {
-		return output, nil
-	}
-	command := strings.TrimSpace(strings.Join(append([]string{name}, args...), " "))
-	detail := strings.TrimSpace(string(output))
-	if detail == "" {
-		return output, fmt.Errorf("%s: %w", command, err)
-	}
-	return output, fmt.Errorf("%s: %w: %s", command, err, detail)
-}
-
 type guestSSHClient interface {
 	Close() error
 	RunScript(context.Context, string, io.Writer) error