script/ptl-tool: introduce interactive backport parity and conflict verification

script/ptl-tool: introduce interactive backport parity and conflict verification

This patch significantly expands `ptl-tool.py` to automate and improve the
backport review process. It adds robust interactive checks to verify that all
commits from an original PR are correctly represented in a backport PR.

Key additions:

* Commit Parity Verification: Analyzes the local Git DAG to ensure all
  cherry-picked commits map properly to the original main PRs, generating a
  visual map of the commits.

* Conflict Simulation: Creates temporary, detached worktrees to dry-run the
  cherry-pick sequence, verifying conflict resolutions dynamically.

* Automated GitHub Reviews: Enables maintainers to open an editor, preview
  markdown drafts, and post `REQUEST_CHANGES` reviews detailing missing commits
  or backport deviations directly to the pull request.

* Interactive Diffing: Provides file-specific 3-pane patch comparisons
  (range-diffs, original patch, backport patch) within the terminal editor during
  conflict investigations.

* New CLI Flags: Introduces `--audit`, `--always-fetch`,
  `--skip-conflict-check`, and `--release-merge` for greater control over the
  script's behavior.

Assisted-by: Gemini
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>