feat(skill): expand shell directives in skill content (#10892)

[rylwin]

Issue for this PR

Closes #10892

Type of change

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

What does this PR do?

Support the !`command` syntax in skills to inject dynamic content at load time, compatible with Claude Code's implementation.

I know there's another PR implementing the same feature, but I think it's stalled out due to feedback needing to be addressed (#10891).

How did you verify your code works?

To manaully QA, I ran opencode with the following prompt (which references a personal skill of mine that uses a shell directive to include a script's help in the skill markdown file):

Load the browser-cli skill. What subcommands are offered?
Do not do anything except load the skill to find the answer.

When I run opencode on dev, I get answers like:

The skill loaded but the content is minimal. Let me check the actual browser-cli script to see what subcommands it offers.

When I run opencode on this branch / PR, I get answers like:

The browser-cli skill offers the following subcommands:
Setup / Lifecycle:
- install - Install native messaging host manifest
- launch - Kill Chrome and relaunch with extension loaded (--headless for Xvfb)
- stop - Kill Chrome and Xvfb
Navigation:
- navigate <url> (aliases: open, goto) - Navigate to URL
- back / forward / reload - Browser history navigation
- url - Get current URL
- title - Get page title
...

Checklist

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

---

Happy to address any feedback. And thanks for opencode!

[github-actions]

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

Potential Duplicate Found:

• PR feat(skill): add dynamic context support with !command syntax #10891: feat(skill): add dynamic context support with !command syntax
  • feat(skill): add dynamic context support with !command syntax #10891

Why they're related:
This PR (#17540) explicitly mentions PR #10891 in its description as an existing PR implementing the same feature. According to the current PR, #10891 is stalled due to feedback needing to be addressed. Both PRs are implementing support for the !command`` syntax to inject dynamic content into skills at load time, which is compatible with Claude Code's implementation. They address the same feature request (#10892).