Add compacted_at_commit field and git commit capture during compaction

- Add compacted_at_commit field to Issue type (bd-405)
- Add database schema and migration for new field
- Create GetCurrentCommitHash() helper function
- Update ApplyCompaction to store git commit hash (bd-395)
- Update compaction calls to capture current commit
- Update tests to verify commit hash storage
- All tests passing

Amp-Thread-ID: https://ampcode.com/threads/T-5518cccb-7fc9-4dcd-ba5a-e22cd10e45d7
Co-authored-by: Amp <amp@ampcode.com>

internal/compact/git.go

@@ -0,0 +1,21 @@
+package compact
+
+import (
+	"bytes"
+	"os/exec"
+	"strings"
+)
+
+// GetCurrentCommitHash returns the current git HEAD commit hash.
+// Returns empty string if not in a git repository or if git command fails.
+func GetCurrentCommitHash() string {
+	cmd := exec.Command("git", "rev-parse", "HEAD")
+	var out bytes.Buffer
+	cmd.Stdout = &out
+	
+	if err := cmd.Run(); err != nil {
+		return ""
+	}
+	
+	return strings.TrimSpace(out.String())
+}