Add CLI command for A2A protocol server management

[Copilot]

What does this PR do?

Adds opencode a2a CLI command for managing the A2A (Agent2Agent) protocol server, completing the command suite alongside acp and mcp.

Implementation:

• New command file src/cli/cmd/a2a.ts with three actions:
  • start (default): Launch A2A server with network options
  • info: Display protocol version and endpoint documentation
  • status: Show active endpoints and server configuration
• Registered in main CLI (src/index.ts) between ACP and MCP commands
• Protocol version (0.3.0) extracted to types.ts constant, used across card generation and CLI output

Usage:

opencode a2a              # Start server
opencode a2a info         # Show protocol details
opencode a2a status       # Display endpoints

Follows established patterns from ACP command (stdio/connection handling) and MCP command (subcommand structure).

How did you verify your code works?

• Unit tests verify command structure and exports
• Documentation updated with CLI usage examples
• Command appears in opencode --help output automatically via yargs registration

Original prompt

I want to expose all opencode capabilities throught Agent2Agent (A2A) Protocol

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[github-actions]

Hey! Your PR title Expose OpenCode capabilities through Agent2Agent (A2A) Protocol 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.