refactor: remove dead code (Commits 3.3 & 3.4)

- Remove packages/agent-runtime/src/tool-stream-parser.old.ts (unused old file)
- All 369 agent-runtime tests pass
- All 13 package typechecks pass

Note: old-constants.ts and deprecated type aliases kept for now as they
are still widely used (52+ imports). Migration to new imports is a
separate task.

Author: brandonkachen