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>
Changed the RunE function of the agent-task command to display help output instead of returning nil, improving user guidance when the command is run without arguments.
Introduces a new `agent-task` command under pkg/cmd/agent with strict OAuth (device flow) token validation. Includes comprehensive tests for token source and host validation, and registers the command in the root command set.
