feat(skills): suggest skill activation

[cniska]

Motivation

The agent relies on its own judgment to decide when to activate a skill. A system-level hint based on the user's message improves activation rates without adding tool call overhead.

Summary

• add trigger keyword map for bundled skills in skill-triggers.ts
• in phasePrepare, scan user message for trigger matches and prepend a directive suggestion to the agent input
• skip suggestions for already active skills
• batch skill-activate tool to accept multiple names in one call
• update tool instruction and output schema for batched activation

Fixes #194