Rewrite building-an-adventure in a direct, guide-appropriate voice

[mmacy]

What changed

Rewrote docs/getting-started/building-an-adventure.md to remove Claudish prose while keeping the guide's richer register. This page is a guide interior, not funnel-top, so it keeps its concept-explaining prose — but every sentence now earns its place.

Cut:

• Meta-commentary about the document structure ("The complete program at the end runs as written...")
• Over-explained edge-key mechanics (which cell keys which edge, the same-edge equivalence)
• Justifying asides ("so you don't have to work out...", "Left unpinned, the engine rolls normally...")
• Verbose enumeration ("optionally stuck or locked, optionally starting open, and optionally gated by an authored condition" → "stuck, locked, or gated")
• Wordy transitions ("To learn about the gate you'd put on this door's requires field, see..." → "For gates, see...")
• Exposition restating what the code shows (3-sentence walkthrough → 1 sentence)
• "Where next" bullets tightened to noun phrases

What didn't change

All code is byte-identical (mechanically verified by diffing every fenced block). All cross-reference links preserved. The guide's richer register is intact in the tree list, keyed areas intro, content spec list, and validate section.

Verification

• pytest tests/test_docs_examples.py — 319 passed, 73 skipped
• mkdocs build --strict — clean