docs(agents): add Laziness Ladder guardrail

[arzafran]

What this does

Adds a "Laziness Ladder" — a six-rung decision gate that agents must clear before writing any code. The ladder encodes the YAGNI / prefer-existing-solutions instinct in an explicit, ordered checklist: does it need to exist? stdlib? platform? installed dep? one-liner? only then write. It also bridges two principles that could feel contradictory: the new Laziness Ladder (skip what isn't needed) and the existing Completeness rule (fully finish what you've decided to build). A new paragraph under Completeness makes the sequencing explicit so agents don't use one as an excuse to violate the other.

Summary

• New ### Laziness Ladder (Before Writing Code) subsection under Guardrails, with 6 ordered rungs and a "Lazy, not negligent" carve-out for trust boundaries, security, a11y, and explicitly requested work
• Bridge paragraph under Completeness Is Cheap clarifying that the ladder decides whether to build; Completeness decides how thoroughly — sequential, not opposed
• External Libraries intro replaced with a back-reference to rungs 2–4 of the ladder instead of restating the same principle in isolation

Test Plan

• 533 existing tests pass (bun test — green)
• Typecheck clean (bunx tsc --noEmit — no errors)
• Reviewed for consistency with adjacent Guardrails sections