feat!: rewrite Waybar modules, remove sotto, and fix GitHub Waybar JSON output

[rbright]

Summary

• add a workspace foundation, deterministic guardrails, CI wiring, shared waybar-core crate, and Renovate config
• rewrite the active Waybar modules (agent-usage, github, linear, schedule) from Go to the workspace crates with package-local tests
• audit and remove the unused sotto module, switch local hooks to prek, and cut the repo over to the workspace-only toolchain/builds
• fix the GitHub Waybar backend on NixOS/Waybar by silencing gh auth status during auth detection so the module emits JSON only
• sync agent-usage pricing and normalization with the upstream CodexBar pricing table for current GPT 5.x and Claude 4.x variants
• tighten the agent instruction router with a concise new-module checklist and clearer progressive-disclosure loading for future module work

Verification

• just ci-check
• just guardrails
• nix develop -c just deps-check
• nix develop -c cargo nextest run --workspace --all-targets
• just nix-build
• nix run 'path:.#github' -- status
• cargo test -p waybar-agent-usage
• git diff --check