feat(plans): implement agents plan rollback command for checkpoint-based rollback

Register the plan rollback subcommand in the CLI.
Implement checkpoint state restoration logic to revert to the saved checkpoint.
Implement decision discarding to remove decisions after the checkpoint timestamp.
Implement plan resume from the rolled-back state to continue execution from the rollback point.
Add BDD tests for rollback functionality to validate end-to-end behavior.
Achieve at least 97% test coverage.
Update CHANGELOG.md with the new feature documentation.

ISSUES CLOSED: #8557

CHANGELOG.md

@@ -257,6 +257,12 @@ The format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
   forward-compatibility.  Added BDD coverage for the stored-JSON path,
   corrupt-JSON fallback, resource-passing, and stub extra-kwargs scenarios. (#828)
 
+- **Plan Rollback Command** (#8557): Implemented `agents plan rollback <plan-id> <checkpoint-id>`
+  command for checkpoint-based plan state restoration. Restores a plan to a previous checkpoint
+  state, discarding decisions made after the checkpoint. Supports `--yes/-y` flag to skip
+  confirmation prompts and `--to-checkpoint` as a named option alternative. Includes
+  comprehensive BDD test coverage (>= 97%) and spec-aligned output formatting with rollback
+  summary, changes reverted, impact analysis, and post-rollback state panels.
 
 - **TDD Issue-Capture Test Activation** (#7025): Replaced 234 bare `@skip` tags
   across 82 Behave feature files with the correct `@tdd_expected_fail @tdd_issue