docs(agents): AGENTS.md overstates input source capabilities (claims touch support that doesn't exist)

[stormmuller]

Summary

AGENTS.md's project overview lists the engine's input capabilities as:

Input: Keyboard, mouse, and touch input handling

src/input/ has keyboard, mouse, and gamepad input sources. There is no TouchInputSource. So the line is wrong twice over — it claims a capability that doesn't exist, and omits one that does.

Raised by @stormmuller while reviewing #580.

Why it matters

AGENTS.md is the primary orientation document for coding agents working on this repo, and CLAUDE.md pulls it in wholesale via @AGENTS.md. An agent reading that line will reasonably assume a touch source exists and design against it — which is exactly what happened in #580, where the UI design initially scoped a TouchInputSource as a prerequisite on the basis of closing a gap the document implied.

Current state

src/input/
  keyboard/input-sources/keyboard-input-source.ts
  mouse/input-sources/mouse-input-source.ts
  gamepad/input-sources/gamepad-input-source.ts

Suggested scope

• Correct the capability line to "Keyboard, mouse, and gamepad input handling".
• Audit the rest of AGENTS.md's Project Overview list against /src while in there — the other bullets (ECS, rendering, physics, audio, animations, particles, asset loading, FSM) should each be confirmed to match what's actually implemented, so this class of drift gets caught in one pass rather than one bullet at a time.
• Consider whether the same overview text is duplicated in README.md or the documentation site and needs the same correction.

Adding an actual TouchInputSource is a separate piece of work and is explicitly not what this issue asks for — it's scoped to making the documentation match reality.