[FEATURE]: Support .local.md variants for instruction files (AGENTS.local.md, CLAUDE.local.md)

[clansty]

Verify there is no existing issue for this

• I have searched the existing issues

Is your feature request related to a problem?

When working in a shared repository, AGENTS.md is committed to git and shared across the team. However, individual developers often want personal instructions (local environment notes, personal coding preferences, machine-specific paths) that should not be committed to the shared repo.

Currently there is no built-in way to have per-user instruction files that are gitignored.

Describe the solution you'd like

Support .local.md variants alongside each instruction file:

• AGENTS.local.md loaded alongside AGENTS.md
• CLAUDE.local.md loaded alongside CLAUDE.md

These files would be loaded additively — both the base file and the .local.md variant are included when present. Users can then add *.local.md to .gitignore for personal instructions.

This follows the same pattern that Claude Code uses with CLAUDE.local.md.

Describe alternatives you've considered

Using the instructions config field to point to a custom file path works, but requires manual configuration per project and the config file itself may be committed.

Related: #11454 proposes .opencode/AGENTS.md for environment-specific instructions — a different approach to a similar problem. This PR uses the .local.md convention which is already established by Claude Code.

Additional context

I have a PR ready: #16108

[github-actions]

Thank you for updating the issue! It now follows the contributing guidelines and looks great. Your feature request is clear and well-structured.