Add a "review mode"

### Describe the feature or problem you'd like to solve

Reviewing PRs or even the changes made by copilot can be a long and difficult process, especially with more code being generated by AI tools.

### Proposed solution

Currently, shift+tab cycles modes like "plan mode". I propose a new "review mode" with the following:
- do NOT allow modifications to the product code
- prefer referring to new changes over full architecture (or how they relate to the full architecture)
- detect code-health issues like `const` in C++, whitespace, etc. Try to match the mannerisms used in the same files. (i.e. function comments?)
- if possible, identify tests to run and provide feedback on that. Should more tests be added? Do the tests pass? Do the tests need to be updated?
- if possible, refer to the actual PR on GitHub. Consider relevant comments in the code area under review. (advanced feature: look at other PRs or recent changes in similar areas, just to be aware of them)

### Example prompts or workflows

_No response_

### Additional context

VS Code lets you look at GitHub PRs. You can see what changes there are, but also have access to AI tooling. I use this all the time to ask questions about the new implementation or how it fits into the larger architecture.

Thinking about how to have a similar experience in the copilot cli would be helpful.

The issue names no files or tests; start by locating the existing Shift+Tab mode handling and plan mode implementation. Define how review mode prevents product-code modifications, focuses feedback on changes, and identifies relevant tests or PR context. Done should include an agreed review workflow and validation of its boundaries.

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