feat:Introducing skills and agents marketplace for opencode CLI

[HomiGrotas]

Issue for this PR

Closes #28696
As Opencode lacks marketplace integration, many users are unable to migrate to Opencode, including my company.
This is the MVP and future capabilities will be developed, such as updating remote marketplaces, and plugins installation.

Type of change

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

What does this PR do?

This PR implements marketplace support from the CLI, using remote (GitLab, GitHub) and local marketplaces.
After installation, a marketplace is being scanned for skills and agents, and those are located in the already existing directories.

Added commands

Commands:
  opencode marketplace install <source>  install a package from a source           [aliases: i, add]
  opencode marketplace uninstall <name>  uninstall a package                   [aliases: remove, rm]
  opencode marketplace list              list installed packages                       [aliases: ls]
  opencode marketplace info <name>       show details about an installed package

Example

opencode marketplace install gh:Nagell/claude-marketplace

How did you verify your code works?

Wrote tests for the marketplace feature and run manually using bun

Screenshots / recordings

irrelevant, feature for CLI only

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 Introducing skills and agents marketplace for opencode CLI 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.