feat: add warm agent-browser session pools

[mikehostetler]

Summary

• add explicit warm session pool support for the AgentBrowser adapter via start_pool/1, stop_pool/1, pooled start_session/1, and the new supervised Jido.Browser.Pool child
• back pooled sessions with NimblePool-managed warm workers plus per-session lease processes, strict recycle-on-release, direct-name pool resolution for supervised pools, and hardened lease ownership/error handling
• thread pooled session support through the actions/plugin layers, recommend the supervised pool path in the docs, and add unit coverage for pool lifecycle and failure recovery

Closes #25.

Testing

• mix test
• mix q