feat(config): add {file:...} interpolation to agent markdown prompts

[daryltucker]

Issue for this PR

Closes # (no issue — feat PR is exempt from issue requirement per check-standards logic)

Type of change

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

What does this PR do?

Support {file:path} syntax in agent markdown config files to include external file contents at load time. File paths resolve relative to the referencing file; absolute paths and ~ expansion also work.

This was originally implemented in the feat/file-interpolation branch (commit f34366bec7) and has been cleaned up, rebased onto dev, and scoped to only the interpolation feature for this PR.

Before: agent and mode prompt fields could only contain literal markdown text. Referencing external content required duplication or separate loading logic.

After: {file:./path/to/file.md} references are resolved at config load time — frontmatter is stripped, circular references and max depth (5) are detected, and errors are reported via HTML comment placeholders + console.warn() without crashing the load.

How did you verify your code works?

• bun test config/ — 194 passed, 3 skipped (pre-existing), 0 failed
• bun typecheck from packages/opencode — clean
• 11 new test cases in interpolate-files.test.ts covering: simple refs, missing files, frontmatter stripping, relative paths, circular references, max depth, same-file branches, recursive inclusion, empty files, multiple distinct refs

Screenshots / recordings

N/A — config/interpolation change, no UI.

Checklist

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

[github-actions]

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

Potential Duplicate Found

PR #34964: feat(opencode): inject file in markdown agents

• feat(opencode): inject file in markdown agents #34964

This PR appears to be addressing the same feature—injecting file contents into markdown agent prompts. It may be an earlier attempt or related work for the same functionality that PR #38379 is implementing with {file:...} interpolation syntax.

[daryltucker]

Tested this for a few weeks and everything seemed to work nicely.
Came back after short vacation, and OpenCode was pretty broken... found the leak issue.
Pulled in the Leak fix, as well as enhancements for architecture changes, and continuing testing.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[github-actions]

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