docs: remove stale pitfalls guide and tidy AGENTS.md

[frendsick]

Summary

• Removes docs/agents/pitfalls.md and its reference in AGENTS.md. Its only entry ("Type enum methods may consume the value") could not be reproduced against current main: injecting the documented chained-.get-into-multiple-enum-methods pattern into a hot compiler path (Type::format) still builds stage1→stage2, self-compiles casa.casa, and runs cleanly with no "unwrap on None". No tracking issue exists and no fix has landed since the doc was added (2026-05-10); the entry itself was labelled "root cause unconfirmed".
• Tidies AGENTS.md: rewords the source-of-truth principle, and moves caveman/adds tdd under the "Agent skills" section.

Verification

• tests/test_bootstrap.sh green on main baseline (self_compilation + fixed_point).
• Repro attempts (two faithful pattern reconstructions) compiled and self-compiled without crashing.