feat: add a new mode (learn mode)

[MichaelYeh507]

Issue for this PR

Closes #10

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a third primary agent, Learn, next to Build and Plan (Tab cycles Build → Learn → Plan; Learn is pinned green).
In Learn mode, every file edit is held until the user answers a comprehension question about the change:

• Before an edit, write, or apply_patch call is applied, a small model call writes one question about the diff, and the user answers it in the TUI.
• A second model call grades the answer. If it passes, the edit applies.
• If it fails, the user sees feedback and gets a new question. Retries are unlimited, but each new question is generated from the last three failed attempts and is asked to break the concept into a smaller, more concrete step.
• Dismissing the question rejects the edit and stops the turn.
• A multi-file apply_patch (used by GPT models) gets one combined question.
• If the check can't run (for example, the change is too large to quiz on, or the model returns malformed output), the edit is rejected with feedback telling the model to make a smaller edit, instead of the tool crashing.

The edit tools are the only gated path, so Learn's system reminder tells the model never to change files through bash or a subagent. This is a prompt-level guard, the same approach plan mode uses. Build and Plan are unchanged.

How did you verify your code works?

I added unit tests for each part of the feature: the Learn agent definition and its reminder, question generation (including how earlier failed attempts shape the next question and how question marks inside code are handled), answer grading (including JSON the model wraps in a code block), the retry loop, the check in the session tools, and the combined question for multi-file apply_patch edits. bun typecheck passes in packages/opencode and packages/core, and CI is green. I also ran it locally with bun dev: I switched to Learn, asked it to edit the README, got a comprehension question first, and the edit was only written after I answered it.

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[drek2991]

Needs a little more clarification on the testing happening; perhaps further integration tests and even maybe going into E2E tests.