docs: add AGENTS.md to orient AI coding agents

[fortuna]

What

Adds an AGENTS.md at the repository root describing the project for AI coding
agents (and, incidentally, for new human contributors).

Why

Intra is a hybrid Java + Go project with a few conventions that are easy to get
wrong and are only discoverable by reading the code and the Gradle build:

• The Go module is named localhost/Intra and lives at the repo root, while the
  Go sources are under Android/app/src/go/. Imports look like
  localhost/Intra/Android/app/src/go/doh, which looks like a mistake but isn't.
• Only backend, intra, intra/split and intra/protect are passed to
  gomobile bind (goSourcePackages in Android/app/build.gradle), and
  anything exported from backend must respect gomobile's type restrictions
  (hence workarounds like the comma-separated ipsStr in NewDoHServer).
• Any Go code opening an external socket must go through
  protect.MakeDialer / protect.MakeListenConfig, or traffic loops back into
  the VPN.
• debug.SetGCPercent(10) in backend/init.go is deliberate.
• Logging goes through the logging package, not log/fmt.

Contents

• Project overview and repository layout
• Architecture diagram and the TUN -> GoVpnAdapter -> backend.ConnectSession
  -> intra.Tunnel flow
• Go and Gradle build/test commands
• Conventions and gotchas (module path, gomobile limits, socket protection,
  strict dependency locking, localization, license headers)
• Commit/PR conventions and a pre-submit checklist

Documentation only; no code changes.

[peterwgnd]

lgtm