feat(context): centralize RepoContext API for git operations (#1102)

Centralizes repository context resolution via RepoContext API, fixing bugs where git commands run in the wrong repo when BEADS_DIR points elsewhere or in worktree scenarios.

internal/compact/git.go

@@ -1,15 +1,29 @@
 package compact
 
 import (
-	"os/exec"
+	"context"
 	"strings"
+
+	"github.com/steveyegge/beads/internal/beads"
 )
 
-var gitExec = func(name string, args ...string) ([]byte, error) {
-	return exec.Command(name, args...).Output()
+// gitExec is a function hook for executing git commands.
+// In production, it uses RepoContext. In tests, it can be swapped for mocking.
+var gitExec = defaultGitExec
+
+// defaultGitExec uses RepoContext to execute git commands in the beads repository.
+func defaultGitExec(name string, args ...string) ([]byte, error) {
+	// name is always "git" when called from GetCurrentCommitHash
+	rc, err := beads.GetRepoContext()
+	if err != nil {
+		return nil, err
+	}
+
+	cmd := rc.GitCmd(context.Background(), args...)
+	return cmd.Output()
 }
 
-// GetCurrentCommitHash returns the current git HEAD commit hash.
+// GetCurrentCommitHash returns the current git HEAD commit hash for the beads repository.
 // Returns empty string if not in a git repository or if git command fails.
 func GetCurrentCommitHash() string {
 	output, err := gitExec("git", "rev-parse", "HEAD")