feat(toolset): improve dedup with sparse reminders and canonical args - #2372

Summary

This PR improves the tool call deduplication system with smarter repeat detection and sparse reminders, makes /clear a true alias for /new in the shell UI, and updates the welcome tip.

1. Tool Call Deduplication Improvements

Problem: The previous deduplication system appended a reminder on every cross-step duplicate call, which could be noisy. It also did not normalize JSON argument key order, causing identical calls with different key orderings to be treated as distinct.

What was done:

• Canonicalize tool arguments by sorting JSON keys for stable dedup comparison
• Track consecutive repeat streaks across steps instead of treating each step independently
• Use sparse reminders: gentle reminder at the 3rd consecutive repeat, stronger detailed reminder at 5th and 8th repeats showing tool name, repeat count, and canonical arguments
• Expand test coverage for canonicalization and sparse reminder thresholds

2. Shell UI Improvements

Problem: /clear and /new had overlapping but slightly different implementations.

What was done:

• Extract shared _do_new_session helper in shell slash commands
• Make /clear a proper alias for /new with consistent session reset behavior
• Update welcome tip to promote Kimi Code download link

Checklist

• I have read the CONTRIBUTING document.
• I have linked the related issue, if any.
• I have added tests that prove my fix is effective or that my feature works.
• I have run make gen-changelog to update the changelog.
• I have run make gen-docs to update the user documentation.