COLLAB_TOKEN_SECRET becomes a root key: every purpose derives its own
HKDF-SHA-256 subkey (deriveTokenKey), and no code path signs with the
root key directly. Collaboration tokens are signed and verified by jose
with HS256 as an explicit allowlist; the sign/verify API turns async at
its three call sites. Unsubscribe tokens move from a purpose-prefix
string to the structural subkey, with a documented dual-verify window
(legacy derivation accepted until 2026-11-01, covering the 90-day TTL
of links in already-sent mail).
The cross-runtime property that justified the homegrown implementation
is now proven by a test: the built CJS and ESM dist artefacts round-trip
tokens in both directions in child processes (jose v6 reaches CJS via
Node's require(esm), pinned Node 22 images). Negative tests cover
cross-purpose subkeys, root-key-signed tokens, alg:none and RS256.
Refs #188 (ADR 0020)
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0168Ph5uBmHm8X28CSVpbpnJ
