feat(cli): --if-exists skip|update|fail for the top 409-conflict writes

Summary

An idempotent-write convention --if-exists skip|update|fail (default fail = today's behavior) on the writes that generate the most HTTP 409 conflicts.

Motivation

Telemetry, 30 days, distinct installs hitting 409: pricing availability create 877, review items add 872, metadata push 596, localizations update 562, review submissions-submit 490, review details-create 455, versions create 437, subscriptions setup 424, age-rating edit 409, bundle-ids capabilities add 398. Together the single largest API-error class (~4.7k installs). Agents retry blindly.

Contract

• skip: exit 0 with an additive receipt field (e.g. action: "skipped") via the registered renderer.
• update: route to the matching update/PATCH with the same inputs where meaningful (versions create -> versions update; localizations create/update routing; review details-create -> details-update; pricing availability create -> availability edit; capabilities add / review items add -> skip semantics).
• Keyed on Apple's exact 409 error codes; other 409s still fail.

PR plan

1. shared helper + receipt + versions create + review details-create
2. localizations update/create + metadata push (stacked)
3. pricing availability create (stacked)
4. bundle-ids capabilities add + review items add (stacked) review submissions-submit, subscriptions setup, age-rating edit analyzed and listed as follow-ups.

Acceptance criteria

• RED httptest per command reproducing the real 409 body
• default behavior byte-identical
• docs/design/if-exists-idempotent-writes.md + command docs regenerated

Partial work: worktree ~/Developer/CLIs/asc-if-exists-core (branch claude/if-exists-core-20260913).