docs: add CURSOR_RIPGREP_PATH gotcha for proof package

[tonyketcham]

Summary of changes

Adds CURSOR_RIPGREP_PATH documentation to AGENTS.md. The @flatbread/proof package uses @cursor/sdk which tries to resolve a bundled ripgrep from @cursor/sdk-linux-x64. In Cloud Agent VMs, this optional dependency isn't installed, but system ripgrep is available at /usr/bin/rg. Setting CURSOR_RIPGREP_PATH=/usr/bin/rg resolves the warning.

The VM update script is also updated to set this env var automatically.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• I added doc comments to any new public exports, and inline comments to any hard-to-understand areas
• My changes generate no new console errors locally
• If applicable, try to include a test that fails without this PR but passes with it

Does this introduce any non-backwards compatible changes?

• Yes
• No

Does this include any user config changes?

• Yes
• No