docs(subscriptions): add v2 web-only target design

[wcjord]

What

Adds the client subscriptions v2 design doc (single web purchase flow, in-app discount codes, device + storefront-country presentation matrix, choreographer-backed entitlement state) and points the v1 doc at it.

Why

Addresses #6964. Org-level design: pangeachat/.github#179. UI implementation tracks separately in #4977.

Testing

N/A — doc-only.

Deploy Notes

None

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 60675fa6-8bb2-4fd0-97f4-733ba35d73cd

📥 Commits

Reviewing files that changed from the base of the PR and between 6c5a8a4 and b3e2592.

📒 Files selected for processing (2)

• .github/instructions/subscriptions-v2.instructions.md
• .github/instructions/subscriptions.instructions.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/subscriptions-v2

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