Retire classic review orchestration

[Jodre11]

Context

Step 2 of the staged orchestration default-flip landed on branch flip/panel-default (commit 70fe83c): panel is now the built-in review orchestration default, with classic kept as a functional explicit opt-in (review_mode = "classic"). This is the deferred step 3 — remove classic entirely once panel-as-default has ridden in the wild with no complaints.

• Step 1: make orchestration config resolution an explicit MUST-READ — shipped as fix(review): make orchestration config resolution an explicit MUST-READ #117.
• Step 2: flip the default classic → panel — this PR (70fe83c).
• Step 3 (this issue): retire classic.

Trigger / revisit

Revisit ~2026-07-31 (roughly one week after the flip merges). Proceed only if no complaints surface about panel-as-default. Depends on the step-2 PR being merged.

Scope (for reference)

Delete the classic code path and its scaffolding:

• The classic phase set in review-core.mjs meta (cross / synth / resample phases).
• The classic branch after the if (mode === 'panel') return (the whole crossAndSynth / synth-stall / resample middle stage).
• The mode constant collapses to unconditional panel.
• The NON_CROSS classic-only simplification noted at review-core.mjs (the "Classic mode is being retired" comment) — the exclusion can be simplified since the panel path never runs cross-review.
• review_mode = "classic" handling in the three synced prose copies (skills/review-gh-pr/SKILL.md, commands/pre-review.md, includes/review-pipeline.md).
• The classic-path tests that this PR made request orchestrationMode: "classic" explicitly — they can be removed or repointed at panel.