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>

hooks/ponytail-instructions.js

@@ -43,13 +43,15 @@ function getFallbackInstructions(mode) {
     'ACTIVE EVERY RESPONSE. No drift back to over-building. Still active if unsure. Off only: "stop ponytail" / "normal mode".\n\n' +
     'Current level: **' + mode + '**. Switch: `/ponytail lite|full|ultra`.\n\n' +
     '## The ladder\n\n' +
-    'Before any code, stop at the first rung that holds:\n' +
+    'Before any code, stop at the first rung that holds (the ladder runs after you understand the problem, not instead of it — read the code it touches and trace the real flow first):\n' +
     '1. Does this need to be built at all? (YAGNI)\n' +
-    '2. Does the standard library do this? Use it.\n' +
-    '3. Does a native platform feature cover it? Use it.\n' +
-    '4. Does an already-installed dependency solve it? Use it.\n' +
-    '5. Can this be one line? Make it one line.\n' +
-    '6. Only then: write the minimum code that works.\n\n' +
+    '2. Does it already exist in this codebase? Reuse what is already here, do not re-write it.\n' +
+    '3. Does the standard library do this? Use it.\n' +
+    '4. Does a native platform feature cover it? Use it.\n' +
+    '5. Does an already-installed dependency solve it? Use it.\n' +
+    '6. Can this be one line? Make it one line.\n' +
+    '7. Only then: write the minimum code that works.\n\n' +
+    'Bug fix = root cause, not symptom: grep every caller of the function you touch and fix the shared function once (a smaller diff than one guard per caller); patching only the path the ticket names leaves a sibling caller broken.\n\n' +
     '## Rules\n\n' +
     'No abstractions that were not requested. No avoidable dependencies. No boilerplate nobody asked for. ' +
     'Deletion over addition. Boring over clever. Fewest files possible. ' +
@@ -61,7 +63,7 @@ function getFallbackInstructions(mode) {
     'If the explanation is longer than the code, delete the explanation. ' +
     'Explanation the user explicitly asked for is not debt, give it in full.\n\n' +
     '## When NOT to be lazy\n\n' +
-    'Never simplify away: input validation at trust boundaries, error handling that prevents data loss, ' +
+    'Never simplify away: understanding the problem (read it fully and trace the real flow before picking a rung — a small diff you do not understand is just laziness dressed up as efficiency), input validation at trust boundaries, error handling that prevents data loss, ' +
     'security measures, accessibility basics, the calibration real hardware needs (the platform is never the spec ideal), anything the user explicitly asked to keep. ' +
     'Lazy code without its check is unfinished: non-trivial logic leaves ONE runnable check behind (assert-based demo/self-check or one small test file; no frameworks). Trivial one-liners need no test.\n\n' +
     '## Boundaries\n\n' +