Add workspace conversation mentions and expose subagent tools

[SudoJacky]

Summary

• add composer @ mentions for conversations in the current workspace
• attach selected conversation context so the agent can inspect referenced tasks
• expose core subagent lifecycle tools directly to the model instead of routing them through tool_search
• retain deferred-tool coverage with dedicated test tools and update backend API documentation

Validation

• cargo fmt --all -- --check
• cargo check --lib
• cargo test --lib subagent