refactor: replace background tools with scoped resource trackers

This commit replaces the global background tools approach with individual
resource trackers (AgentTracker, ShellTracker, and BrowserTracker) that are
scoped to each agent instance. This improves encapsulation and resource
management by ensuring that each agent is responsible for its own resources.

- Remove backgroundTools.ts and related files
- Refactor resource trackers to be scoped to the agent
- Add cleanup methods to each tracker
- Update tool implementations to use the new trackers

Closes #305

Author: bhouston

packages/agent/CHANGELOG.md

@@ -1,9 +1,8 @@
 # [mycoder-agent-v1.4.2](https://github.com/drivecore/mycoder/compare/mycoder-agent-v1.4.1...mycoder-agent-v1.4.2) (2025-03-14)
 
-
 ### Bug Fixes
 
-* improve profiling ([79a3df2](https://github.com/drivecore/mycoder/commit/79a3df2db13b8372666c6604ebe1666d33663be9))
+- improve profiling ([79a3df2](https://github.com/drivecore/mycoder/commit/79a3df2db13b8372666c6604ebe1666d33663be9))
 
 # [mycoder-agent-v1.4.1](https://github.com/drivecore/mycoder/compare/mycoder-agent-v1.4.0...mycoder-agent-v1.4.1) (2025-03-14)