fix: comprehension-first guard + reuse rung (#245, #217) (#253)

#245 "Dangerously lazy": add an operational "fix the root cause, not the
symptom" directive — grep every caller of the function you touch and fix the
shared function once (the smaller diff). Validated on the agentic benchmark: on
a shared-helper bug-fix trap, baseline fixes the root cause 1/6 while ponytail
does 6/6 on both Sonnet 4.6 (the model the issue was filed on) and Opus 4.8,
verified by reading the produced code. Plain prose ("trace the flow") did not
move it; the actionable, lazy-framed directive did.

#217 "Missing rung": add ladder rung 2 "Already in this codebase? Reuse it,
don't re-write it." Propagated across SKILL.md, AGENTS.md, all agent mirror
copies, the hook fallback, and both READMEs (check-rule-copies passes).

Benchmark: 4 new deterministic quality-tier tasks (reuse-slug, reuse-money,
trace-transfer, trace-amount) with selftest-proven good/bad refs; harness gains
multi-file seed support in --selftest, distinctive-behaviour reuse detection,
and counts in-file __main__/demo() self-checks as test LOC (not source bloat)
for surgical tasks. Full writeup in
benchmarks/results/2026-06-22-issue-245-217-comprehension.md.

Also carries the in-progress todo-null benchmark task already present in the
working tree.

Co-authored-by: Dietrich Gebert <dgebert@Dietrichs-MacBook-Pro.local>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -83,13 +83,16 @@ Before writing code, the agent stops at the first rung that holds:
 
 ```
 1. Does this need to exist?   → no: skip it (YAGNI)
-2. Stdlib does it?            → use it
-3. Native platform feature?   → use it
-4. Installed dependency?      → use it
-5. One line?                  → one line
-6. Only then: the minimum that works
+2. Already in this codebase?  → reuse it, don't rewrite
+3. Stdlib does it?            → use it
+4. Native platform feature?   → use it
+5. Installed dependency?      → use it
+6. One line?                  → one line
+7. Only then: the minimum that works
 ```
 
+The ladder runs *after* it understands the problem, not instead of it: it reads the code the change touches and traces the real flow before picking a rung. Lazy about the solution, never about reading.
+
 Lazy, not negligent: trust-boundary validation, data-loss handling, security, and accessibility are never on the chopping block.
 
 ## Install