feat: adopt pairwise-tests (PICT combinatorial) + cross-reference from existing testing skills

Forked verbatim from omkamal/pypict-claude-skill (MIT). LICENSE preserved.
Renamed from `pict-test-designer` to `pairwise-tests` — technique-first
naming so users invoking "pairwise" or "combinatorial" find it; PICT
remains the tool under the hood.

Bundle (skill-runtime only):
  pairwise-tests/SKILL.md                    (renamed, description rewritten)
  pairwise-tests/LICENSE                     (MIT, preserved)
  pairwise-tests/references/pict_syntax.md
  pairwise-tests/references/examples.md
  pairwise-tests/scripts/pict_helper.py      (Python CLI for model gen / output fmt)
  pairwise-tests/scripts/README.md

Upstream's repo-level docs (README, QUICKSTART, CONTRIBUTING, etc.) and
`examples/` dir (ATM + gearbox walkthroughs — useful as reading, not as
skill-runtime) omitted from the fork. Attribution footer added.

Cross-references so /add-tests naturally routes to /pairwise-tests when
warranted:

- add-tests/SKILL.md Phase 2 step 8: if a function in scope has 3+ parameters
  each taking multiple values, surface `/pairwise-tests` to the user before
  proposing normal category coverage. Default continues with /add-tests;
  user picks pairwise explicitly.
- claude-rules/testing.md: new "Combinatorial Coverage" section after the
  Normal/Boundary/Error categories. Explains when pairwise wins, when to
  skip (regulated / provably exhaustive contexts, ≤2 parameters, non-
  parametric testing), and points at /pairwise-tests.
- languages/python/claude/rules/python-testing.md: new "Pairwise /
  Combinatorial for Parameter-Heavy Functions" subsection under the
  parametrize guidance. Explains the pytest workflow: /pairwise-tests
  generates the matrix, paste into pytest parametrize block, or use
  pypict helper directly.

Mechanism note: cross-references are judgment-based — Claude reads the
nudges in add-tests/testing/python-testing and acts on them when appropriate,
not automatic dispatch. Craig can still invoke /pairwise-tests directly when
he already knows he wants combinatorial coverage.

Makefile SKILLS extended; make install symlinks /pairwise-tests globally.

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