docs: add AGENTS.md and pull-requests skill

[mtojek]

Let this be the starting point for AI agents in this repo — a shared set of guidelines and skills to help them work effectively with the codebase. Just like README, these files need to be maintained as the project evolves.

Changes

• AGENTS.md — main agent guidelines document with:

  • Tone & relationship rules (critical peer reviewer, no sycophancy)
  • Foundational rules (YAGNI, smallest changes, match style)
  • Essential commands table (make targets)
  • Architecture overview of AI Bridge as a gateway
  • Go patterns, streaming code guidelines, commit style
  • Common pitfalls table

• .mux/skills/pull-requests/SKILL.md — PR workflow skill with:

  • Lifecycle rules (check existing PR, default to draft, never auto-merge)
  • PR title/description conventions
  • CI follow-up workflow (gh pr checks --watch, gh run view --log-failed)
  • Pre-push validation checklist

• .gitignore — added AGENTS.local.md for local overrides

Context

Adapted from:

• https://github.com/coder/coder/blob/main/AGENTS.md
• https://github.com/coder/vscode-coder/blob/main/AGENTS.md
• https://github.com/coder/coder/blob/main/.agents/skills/pull-requests/SKILL.md

Skipped items not applicable to this repo: frontend, database, git hooks, swagger, OAuth2.