Replaces linear search with a map for associating sessions with pull requests, improving performance and simplifying code in hydrateSessionPullRequests.
Introduces a new 'list' subcommand under agent-task for listing agent tasks. Implements a Copilot API client for fetching agent sessions and hydrating them with pull request data. Updates PullRequest and PRRepository types to support new fields. Adds dependencies for msgpack and tagparser.
Co-Authored-By: Babak K. Shandiz <babakks@github.com>
