chore(proto): drop pre-v1 reserved statements for Requester.billing_ref

[EugeneDymo]

Pre-v1 the wire contract is not frozen and there are no released consumers to protect, so a removed field's number returns to the free pool instead of being reserved. Matches the earlier Transaction{Request,Response} change; reserved becomes the tool for field removals once v1.0.0 is tagged.

The conformance pin keeps only the design invariant (Requester never regains a caller-written billing_ref field) and no longer freezes the field number.

[legendko]

One documentation gap

website/src/content/docs/reference/changelog.mdx — stale mirror

proto/CHANGELOG.md was updated; its hand-maintained website mirror was not. reference/changelog.mdx must mirror proto/CHANGELOG.md and must be updated after meaningful proto changes.

Lines 36–37 still read (stale):

billing label on Requester is gone; field number 5 and the name are
reserved so they can never be reused with a different meaning. Nothing read

This directly contradicts the branch's own thesis and the updated proto/CHANGELOG.md, which now reads:

billing label on Requester is gone; the field is deleted outright with no
reserved statement — pre-v1 the number returns to the free pool, and
reserved becomes the tool for field removals only once v1.0.0 is tagged.
Nothing read

Fix: replace lines 36–37 of changelog.mdx with the four-line proto/CHANGELOG.md wording above (lines 53–56 there). Mechanical sync; no other divergence in that section.