feat(code-review): add thematic triage grouping

[jblwilliams]

Summary

Adds optional thematic grouping to ce-code-review so related findings are easier to triage together instead of as one long flat list.

The existing severity tables and stable finding IDs stay as the source of truth, but reviews can now include a short Triage Groups section that explains which findings are related, why they matter as a theme, and what order they should be handled in.

Why

On larger reviews, the finding count can easily get into the double digits. A lot of those issues are often related, and one design decision or fix can resolve several of them at once. Grouping gives developers more context about the shape of the feedback so they do not have to work through 20 or 30 individual findings as if each one were independent.

Changes

• Adds grouping controls: grouping:auto, grouping:off, and grouping:always
• Adds triage group output for interactive and headless review modes
• Keeps stable finding IDs consistent across groups, severity tables, and residual work
• Prunes group references after validation drops findings
• Updates the review output template and contract coverage

[tmchow]

@jblwilliams I'd like to see a few examples of output if you can provide it.