docs(release-notes): add product surface update index

[drewstone]

Summary

• add release-notes hub at /release-notes
• add June 29 product-surface release note for Blueprint Agent, Agent Dev Container / Sandbox, Router, Browser Agent, Audit Agent, and protocol contracts
• add release note metadata entry

Verification

• yarn install --immutable
• yarn format:check
• yarn build
• live curl: https://docs.tangle.tools/release-notes 200
• live curl: https://docs.tangle.tools/release-notes/2026-06-29-product-surfaces 200
• live browser pass: desktop/mobile release notes pages returned 200 with no horizontal page overflow

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[tangletools]

✅ Auto-approved drewstone PR — 8e93aea9

This PR was opened by the trusted drewstone account.
The full PR reviewer audit still runs separately and will publish findings if it detects issues.

_{tangletools · auto-approval · reason: drewstone_author · 2026-06-29T13:55:49Z}