2.x gap: expose JS stamp and Claude hook helper APIs

[willwashburn]

From the Rust 2.x gap catalog.

Context:

• Deleting @relayburn/ledger removes JS helper APIs such as stamp() and buildClaudeHookSettings().
• Rust embedders can use relayburn_sdk::{RawLedger, Stamp, StampSelector} directly, but Node embedders do not yet have a 2.x facade for these helpers.

Acceptance:

• Decide whether these helpers belong in @relayburn/sdk 2.x, another Node package, or should be replaced by documented CLI-only workflows.
• If kept, expose typed Node APIs and tests for stamping and Claude hook settings.
• Update README migration/composability docs accordingly.