docs(testing): make testing:plan's test-type classification table the cited SSOT

[kyle-sexton]

What-test-type-per-change knowledge lives in three places (design test-seam thread, architect/plan Step 2, testing:plan table). Cite one.

Origin: skill-name audit + interview, contract at docs/topics/shadowed-skill-renames/PLAN.md (PR #256).

[kyle-sexton]

This was generated by AI during triage (historic normalization, rule 8g).

Sites + fix pattern: plugins/testing/skills/write/SKILL.md + plugins/testing/README.md (the two files carrying the current test-type table), plus the design test-seam thread doc and planning:plan Step 2 (the two other sites the issue names as duplicating the same knowledge) — pick the testing:plan table as canonical per the issue's own framing, point the other two at it instead of restating.