Decide long-term home for the bootstrap authentication hook specification

[coderabbitai]

Summary

The bootstrap server authentication module currently references the authentication hook specification at:

• https://github.com/holochain/sbd/blob/main/spec-auth.md

That specification is still relevant to the current implementation, but its current location is no longer ideal now that tx5/SBD support has been removed from Kitsune2. We should decide whether to keep linking to that external document, move the spec into this repository, or publish it somewhere else that better reflects its ongoing ownership and usage.

Why this follow-up is needed

This came up during review of the tx5/SBD removal work in #560. The code in crates/bootstrap_srv/src/auth.rs still implements and documents behavior based on this spec, but the spec currently lives in the holochain/sbd repository, which may no longer be the best long-term location.

Affected areas

• crates/bootstrap_srv/src/auth.rs
• Any other documentation that references the authentication hook protocol/specification
• Potentially repository docs if the spec is relocated here

Questions to resolve

• Should the authentication hook specification remain in holochain/sbd?
• If not, where should the source-of-truth live?
• Should it be copied or moved into holochain/kitsune2?
• If it is moved, what should the stable link/reference path be?
• Are there any other repos or docs that need their links updated?

Acceptance criteria

• A clear decision is made about the canonical home of the authentication hook specification.
• If relocation is chosen, the spec is added to the appropriate location and linked from the relevant code/docs.
• Existing references are updated to point to the canonical location.
• Any ownership/maintenance expectations for the spec are documented.

Backlinks

• PR: refactor: remove tx5 #560
• Review comment: refactor: remove tx5 #560 (comment)
• Requested by: @ThetaSinner