[codex] Set Rust-first project default

[donbeave]

Summary

Adds a concise development stance to the README: Holla should prefer Rust for project-owned commands, probes, release tooling, parsers, and durable automation.

What ships

• Documents Rust-first project guidance in the main README.
• Allows narrow exceptions when another ecosystem is the natural fit.

Verify locally

git clone https://github.com/tailrocks/holla "$HOME/Projects/tailrocks/test/pr-rust-first/holla"
cd "$HOME/Projects/tailrocks/test/pr-rust-first/holla"
git fetch origin codex/rust-first-guidance
git checkout codex/rust-first-guidance

Review README.md and confirm the guidance is clear.

Migration notes

None.