chore: remove skill metadata from command approval payloads

[bolinfest]

Why

This is effectively a follow-up to #15812. That change removed the special skill-script exec path, but skill_metadata was still being threaded through command-approval payloads even though the approval flow no longer uses it to render prompts or resolve decisions.

Keeping it around added extra protocol, schema, and client surface area without changing behavior.

Removing it keeps the command-approval contract smaller and avoids carrying a dead field through app-server, TUI, and MCP boundaries.

What changed

• removed ExecApprovalRequestSkillMetadata and the corresponding skillMetadata field from core approval events and the v2 app-server protocol
• removed the generated JSON and TypeScript schema output for that field
• updated app-server, MCP server, TUI, and TUI app-server approval plumbing to stop forwarding the field
• cleaned up tests that previously constructed or asserted skillMetadata

Testing

• cargo test -p codex-app-server-protocol
• cargo test -p codex-protocol
• cargo test -p codex-app-server-test-client
• cargo test -p codex-mcp-server
• just argument-comment-lint