chore: set cache control markers on tools

[dannykopping]

Injected tools are not being cached currently, leading to unnecessarily inflated costs.

This PR follows guidance from https://docs.claude.com/en/docs/build-with-claude/prompt-caching#prompt-caching-examples:

The cache_control parameter on the last tool definition caches all tool definitions.

[dannykopping]

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• chore: set cache control markers on tools #38 👈 (View in Graphite)
• feat: runtime-configurable upstream request logging #37
• main

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[dannykopping]

Closing this out; trying to second-guess the client's caching decisions is a recipe for pain.
The original issue of inflated costs was thought to have been caused by lack of tool caching; this was actually incorrect. We were in fact adding coder's MCP tools twice (both via bridge's auto-injection and configuring Claude Code to do it client-side).