feat(opencode): Nested custom tool files

[dbpolito]

Issue for this PR

Closes #15155

Type of change

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

What does this PR do?

Allow custom tools to be organized in subdirectories under .opencode/tools/. Tools in folders are automatically prefixed with the folder name (e.g., .opencode/tools/github/pr.ts becomes github_pr, and index.ts in a folder uses the folder name only).

This enables better organization of related tools without name collisions.

How did you verify your code works?

Added test coverage

Screenshots / recordings

Checklist

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

[github-actions]

Hey! Your PR title Opencode nested custom tools 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]

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