docs: add AGENTS contribution guide

[ibetitsmike]

What changed

• Added a new AGENTS.md at the repository root as an AI-agent contribution guide.
• Documented repository-specific context for contributors:
  • project goals and technology stack
  • key files/directories and architecture flow
  • essential build/format/lint/test/clean/dev commands
  • important packaging and proto-maintenance scripts
  • coding patterns and anti-patterns observed in the codebase
  • code style expectations from .editorconfig
  • commit and PR expectations for validation and rollout notes

Why

• Gives AI contributors a single, concrete source of truth for how to work safely and consistently in this repo.
• Reduces onboarding and prevents repeated mistakes by codifying existing conventions and constraints.

How it was validated

• ./scripts/run-linux-dev.sh --help (confirmed documented flags/usage)
• bash -n scripts/run-linux-dev.sh Packaging.Linux/build-release-packages.sh Packaging.Linux/build-aur-source.sh Packaging.Linux/build-deb.sh
• dotnet restore Coder.Desktop.sln -v q
• dotnet test Tests.CoderSdk/Tests.CoderSdk.csproj --no-restore -v q

All commands completed successfully in this workspace.

Risk / rollback

• Risk is low: documentation-only change; no runtime or packaging behavior modifications.
• Rollback is straightforward: revert commit cdba7b6 if needed.