Add analogous-bugs skill for finding sibling bugs across parallel constructs

[SanderMuller]

Following your suggestion on #6090: extracting part of the claude-fix-issue workflow prompt into a skill that lives here.

This is Step 4 of that prompt (hunting for analogous bugs), the part not already covered by the regression-test skill that Step 2 delegates to. It captures the parallel-construct axes (the accessory string and array type families, the is_* narrowing functions, the class-member and callable axes, the TypeSpecifier truthy/falsey contexts, PHPDoc tags, and the rules and reflection families) plus the procedure for probing each sibling with a failing test.

With it in .claude/skills/, the workflow's inline Step 4 can shrink to a reference:

## Step 4: Hunt for analogous bugs and expand the fix

Follow the `analogous-bugs` skill (`.claude/skills/analogous-bugs/SKILL.md`): it lists the
parallel-construct axes to probe and the procedure for finding sibling bugs and fixing them.
This is the step that distinguishes a good fix from a great one, do not skip it. Track the
axis and the suspects in your TodoWrite list. There is no upper bound on diff size for this step.

It complements the "look for the same bug in adjacent code" note in CLAUDE.md with the concrete catalogue, and defers to regression-test for writing the tests.