feat(skills): absorb falsifiability discipline into writing-good-tests

Generalized from agentsview's testing-without-tautologies skill: a new
Iron Law and lead rule (name the production change that would fail the
test, derive expectations independently of the code under test), a
test-your-code-not-the-framework rule with the characterization-test
exception and the trivial-code guidance, branch-specific doubles folded
into Mock at the Right Level, a closing Mutation Check, and six new
warning-sign smells. Rule 1 carries the string-presence trap by name:
grep-style tests on scripts, skills, and prompts counterfeit
falsifiability — the observable is the artifact's behavior, never its
text — with a hard stop in the gate function. Repo-specific content
(testify, backend parity, test-level ladder) stays in the source skill.
Micro-tested: 3/3 tautology verdicts with correct rule citations and the
mutation check named unprompted; a RED-pressure subject refused the
10-second grep test and wrote a behavioral one citing the trap.

skills/test-driven-development/SKILL.md

@@ -204,6 +204,7 @@ Next failing test for next feature.
 | **Shows intent** | Demonstrates desired API | Obscures what code should do |
 
 When writing or changing any test, read [writing-good-tests.md](writing-good-tests.md) for the rules that keep tests honest:
+- Name the production change that would make the test fail — before writing it
 - Assert on real behavior, never on mock behavior
 - Keep test-only code in test utilities, out of production classes
 - Understand a dependency's side effects before mocking it