feat: add capture_screenshot and capture_scene_screenshot tools (fixes #88)

[Yoshyaes]

Summary

• Adds capture_screenshot and capture_scene_screenshot MCP tools that return a rendered Godot scene as a base64 PNG image, closing Feature Request: Visual Debugging / Automated Playtest Capture Tool #88
• Routes through the existing godot_operations.gd bundled script (no new GDScript files, no autoloads, no editor plugins)
• Runs Godot without --headless so the renderer produces real pixels; fixes the Y-axis inversion from ViewportTexture with img.flip_y()
• Output always goes to a secure OS temp path (user never controls it); temp file is cleaned up after every call

Why this over the existing PRs

PR	Problem
#62 / #65	Require a ScreenshotManager autoload in the user's project, breaking the no-plugin architecture
#100	Standalone GDScript outside godot_operations.gd; Y-axis flip bug; user-controlled output path

This PR follows the same bundled-script pattern as every other tool and addresses all open review comments from #100.

How to test

# Build
npm install && npm run build

# Structural test (no Godot needed)
npm test  # 4 pass, 3 skipped (skipped = Godot not installed)

# Full test with Godot installed
GODOT_PATH=/path/to/godot npm test  # all 4 pass

# Manual CLI smoke test
godot --path tests/fixtures/capture-test-project \
      --script src/scripts/godot_operations.gd \
      capture_scene_screenshot \
      '{output_path:/tmp/test.png,scene_path:res://scenes/main.tscn}'
# Open /tmp/test.png -- should be a solid red 1280x720 frame

Rendering context note

These tools require a real display (not --headless). On a local dev machine (macOS, Windows, Linux desktop) they work out of the box. On headless Linux wrap Godot in xvfb-run -- see the README Visual Debugging section for the wrapper script pattern.

What is not in this PR

record_playtest (capture a frame sequence over N seconds) is scoped to a follow-up PR to keep this one reviewable.

---

Fixes #88