Use rules (.sourcegraph/*.rule.md) in chat and auto-edit

[sqs]

(Experimental and WIP.)

For our code review agent (announced in EAP in https://sourcegraph.com/blog/introducing-enterprise-ai-agents), we're introducing rules that are *.rule.md files placed in .sourcegraph/ dirs in your codebase. The code review agent uses these rules to find and fix issues at code review.

This PR makes it so that Cody in the editor also reads the rules and uses them in chat, auto-edit, and inline edit for higher quality and consistency.

Docs and blog post coming soon after we've merged this and tested it in our own dev workflows.

• Closes https://linear.app/sourcegraph/issue/CODY-4829.
• Depends on https://github.com/sourcegraph/sourcegraph/pull/3305.
• See related use rules in auto-edit #6930, which makes rules work in auto-edit.

cody-rules-chat-autoedit.mp4

TODOs

• Editor (not sidebar) chat does not show the rules for the file in the other pane because it's not active
• Node.js FS errors when looking for .sourcegraph

Test plan

TODO