fix(sync): canonicalize dbPath to fix filepath.Rel error (GH#959) (#960)

Problem:
- When dbPath is set to relative fallback (".beads/beads.db"),
  findJSONLPath() returns a relative path
- daemon_sync_branch.go calls filepath.Rel(absolutePath, relativePath)
  which fails with: "Rel: can't make .beads/issues.jsonl relative to ..."

Solution:
- Canonicalize dbPath at assignment in main.go:471 (source fix)
- Add defensive guard in findJSONLPath() (defense-in-depth)
- Use utils.CanonicalizePath() for OS-agnostic handling
  (symlinks, case normalization on macOS)

Testing:
- Add TestFindJSONLPath_RelativeDbPath (tracer bullet for bug)
- Add edge case tests for BEADS_JSONL and empty dbPath
- All sync mode tests pass including daemon E2E

cmd/bd/main.go

@@ -468,7 +468,10 @@ var rootCmd = &cobra.Command{
 					os.Exit(1)
 				}
 				// For import/setup commands, set default database path
-				dbPath = filepath.Join(".beads", beads.CanonicalDatabaseName)
+				// Invariant: dbPath must always be absolute for filepath.Rel() compatibility
+				// in daemon sync-branch code path. Use CanonicalizePath for OS-agnostic
+				// handling (symlinks, case normalization on macOS).
+				dbPath = utils.CanonicalizePath(filepath.Join(".beads", beads.CanonicalDatabaseName))
 			}
 		}