feat(opencode): add enforced learn mode

[vmiloserdov]

Issue for this PR

Closes #15

Type of change

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

What does this PR do?

User story

As a student completing a coding assignment, I want to use AI to understand the codebase while writing the assignment myself, so that I can strengthen my software engineering skills.

The existing Plan mode relies on a system prompt. That is useful guidance, but it cannot guarantee that a tool call will not modify the working tree. This PR adds Learn mode as an enforced read-only alternative for students who want explanations, code navigation, and research without allowing OpenCode to make changes for them.

Changes

• Adds Learn as a primary agent, so it appears in the existing agent selector and participates in the normal mode-switching flow.
• Gives Learn a focused prompt that frames it as a research and explanation mode.
• Restricts Learn's tool registry to read, glob, grep, webfetch, and websearch. Write tools, task delegation, custom tools, plugin tools, and MCP tools are omitted before the model can call them.
• Rejects both direct shell input and configured command expansion when the active agent is Learn. This protects the two session paths that can otherwise invoke a command outside the normal tool list.
• Styles Learn consistently in the app and session UI.
• Keeps the local .devcontainer setup out of version control via .gitignore; it is only for my local container workflow and is not part of this feature.

I did not use a "revert after the fact" strategy such as git reset --hard. A reset could discard the student's own uncommitted work. Learn instead prevents mutation-capable paths from being exposed or started.

How did you verify your code works?

Automated verification:

• bun typecheck from packages/opencode
• bun test test/agent/agent.test.ts test/tool/registry.test.ts test/session/prompt.test.ts from packages/opencode

The tests cover:

• Learn being available as a primary agent with only the intended permissions.
• The read-only registry containing exactly the five allowed tools.
• A Learn session rejecting a direct shell write attempt before it runs.
• A Learn session rejecting a configured command that would expand into a shell write attempt.

Manual verification:

• Started OpenCode, selected Learn from the existing mode selector, and asked questions about repository code.
• Asked Learn to create a file and to override its restrictions. It refused, and no file was created.
• Entered shell mode with ! and attempted a command that writes a file. Learn rejected the command before execution.
• Confirmed the working tree remained unchanged with git diff --exit-code and git status --short.
• Switched back to Build mode and confirmed that normal editing remains available there.

Screenshots / recordings

The Learn option uses the existing mode selector and agent styling. I will attach a short terminal recording of the read-only prompt and shell checks before this draft is marked ready for review.

How it looks:

Rejecting edit prompt:

Rejects the shell commands that are run in Shell mode (! in the front of the prompt):

Accepting the allowed prompt:

Link to GDrive recording:
https://drive.google.com/file/d/1gg9rFkDplu-3DVvuWCNoM--lAjMTzpln/view?usp=drive_link

Checklist

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

[yswcyswc]

The read-only protections look solid overall. I left one integration question about allowing read-only learning commands within Learn mode.

[yswcyswc]

Should Learn mode block every slash command here, or only commands that can modify the repository? As written, this appears to prevent read-only learning commands such as /group and /newcomer from running while Learn mode is active. Since these features are intended to integrate with Learn mode, it may be useful to allow explicitly read-only commands while continuing to block shell expansions and mutation-capable commands.

[vmiloserdov]

Great note. Yes, Learn mode will allow a specific list of commands, and I think this part should be reserved for another PR, when we move onto the next stage of the project, which is the integration of the Learn Mode.