feat(opencode): Add inline $skill invocations

[mario-loomia]

Issue for this PR

#15617

Type of change

• New feature

What does this PR do?

This adds inline $skill support to the prompt composer.

Typing $ surfaces available skills in autocomplete, and selecting one inserts the skill into the prompt.

How did you verify your code works?

• Ran bun test test/tool/bash.test.ts
• Ran bun run typecheck
• Verified the inline skill flow locally in the app

Screenshots / recordings

Checklist

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

[github-actions]

Hey! Your PR title TUI: Add inline $skill invocations 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.