docs: add bun-only warning to contributing guide

[Zhanbingli]

Summary

This PR refines the CONTRIBUTING.md to address common setup hurdles for new contributors.

Changes

• Explicit Bun Requirement: Added a warning to strictly use Bun (v1.3+) and avoid npm/pnpm to prevent protocol errors (e.g., EUNSUPPORTEDPROTOCOL with catalog:).
• Monorepo Navigation: Clarified that core CLI development can be done directly within packages/opencode.

Motivation

As a new contributor, I encountered several "dead loops" during the initial setup:
Attempting to use npm led to errors due to the catalog: protocol.
This update aims to flatten the learning curve for others.

Verification

• Verified the updated instructions manually by re-cloning and following the new guide.
• Confirmed that bun install works perfectly while npm fails as expected.

[github-actions]

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No duplicate PRs found

[github-actions]

Closing this pull request because it has had no updates for more than 60 days. If you plan to continue working on it, feel free to reopen or open a new PR.