feat(opencode): inject file in markdown agents

[viktorashi]

Issue for this PR

Adresses relevant to #26434
Folks already talking about this

• fix(config): honor frontmatter prompt when agent .md body is empty #26489
• feat: {env:MY_VAR} support in the frontmatter of agent/command markdown files (resolves #5054, #5082) #5092

Type of change

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

What does this PR do?

• resolve {file:...} and {env:...} templates for markdown-backed agent prompts using the same substitution path as JSON config
• honor markdown agent body first, but fall back to frontmatter prompt when the body is empty
• keep behavior aligned with the earlier frontmatter/env work while fixing the remaining body/template gap

If you paste a large clearly AI generated description here your PR may be IGNORED or CLOSED!

How did you verify your code works?

• bun test test/config/config.test.ts in packages/opencode
• bun typecheck in packages/opencode
• push hook also ran bun turbo typecheck

Screenshots / recordings

If this is a UI change, please include a screenshot or recording.

Checklist

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

If you do not follow this template your PR will be automatically rejected.

[github-actions]

Hey! Your PR title Feat/inject file in markdown agents doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, here are the potentially related PRs:

1. PR fix(config): error on missing {env:VAR} in config templates #33854 - fix(config): error on missing {env:VAR} in config templates

   • fix(config): error on missing {env:VAR} in config templates #33854
   • Related because it addresses {env:...} template substitution in config, which is directly mentioned in this PR as part of the work being aligned with

2. PR feat(smart-rules): add context-aware rule injection system with Claude Code compatibility #10090 - feat(smart-rules): add context-aware rule injection system with Claude Code compatibility

   • feat(smart-rules): add context-aware rule injection system with Claude Code compatibility #10090
   • Could be tangentially related to agent prompt injection, though likely a different domain

The PR description also mentions these related issues:

• Bug: agent .md prompt: frontmatter field is silently overridden by markdown body, producing empty prompt and silent fallback to default agent prompt #26434 (the main issue being addressed)
• fix(config): honor frontmatter prompt when agent .md body is empty #26489
• feat: {env:MY_VAR} support in the frontmatter of agent/command markdown files (resolves #5054, #5082) #5092

These are the most relevant existing PRs that touch similar areas of template substitution and agent configuration. PR #33854 is particularly relevant since it deals with the same {env:...} template syntax handling.

[github-actions]

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