refactor(reflect): rename review candidate to target

[Donmagelas]

What

Rename the existing session-level Reflect review candidate type to reviewTarget.

Why

#532 introduces generated fact/skill candidates. Keeping the current session selection type named candidate makes the package ambiguous before the generated candidate types land.

How

• Rename internal/reflect/candidates.go to review_targets.go.
• Rename the internal session review type and helpers to reviewTarget / unreviewedTarget / sortReviewTargets.
• Rename local loop variables and the trace attribute to review_target_count.
• Add a compile-time test guard that listUnreviewed returns []reviewTarget.

Test Plan

• mise exec -- go test -count=1 ./internal/reflect

Refs

• Refs feat: add Reflect fact and skill candidate generation #532