Add GovernanceEnvelope proto message and embed it in
GetCanonicalHeadResponse, CommitResponse, and
MergeToCanonicalResponse. The envelope carries the git ref,
actor DID, accord hash, and ceremony ID so consumers can
forward governance context to Chronicle and Quartermaster
without reconstructing it.
Wire-compatible: the envelope field is optional (proto3 default),
so existing consumers that don't read it are unaffected.
Signed-off-by: Tyler King <tking@guildhouse.dev>