release prep: 0.21.0

[yonidavidson]

Version bump only — main already carries the tree this releases. release-cut.yml runs after this merges.

Ships the fixes from a field report of an 18h multi-agent orchestration session, where the bus worked as a notification layer but not as a delivery layer:

• Alias drift silently splits the mailbox: inbox reads a different (empty) box mid-session #157 the session alias no longer drifts mid-session — a wrapper process used to move the agent to a different, empty mailbox while mail kept arriving at the old one
• Reads consume before delivery: an interrupted inbox destroys the messages it never printed #158 mail is delivered before it is consumed; an interrupted read costs a duplicate, never a message
• Consuming a mailbox is O(messages) remote round-trips — inbox times out where peek is instant #159 consuming a mailbox is one round trip instead of one per message (inbox used to time out where peek was instant)
• No way to clear mail you have already read — peek cannot ack, so the unread count never drops #160 new agentcomm ack <id…> | --all clears mail read some other way, so the unread count can actually drop
• Subagents silently inherit the parent's alias: they drain its inbox and overwrite its status #161 two live processes on one mailbox (a subagent and its parent) announce themselves before one drains the other
• send reports success with no signal that the recipient is not reading #162 send reports when a recipient is not reading; agents/network carry unread depth and last-read age
• The daemon re-reads the whole bus history on every poll — bound the warm mirror to a recent window #167 the daemon keeps a week of history warm instead of the whole archive, every poll

SDK note (pre-1.0, so a minor): Snapshottable.snapshot now takes a body filter and returns {keys, bodies}; Batchable (moveMany) is a new optional capability. Both are optional interfaces only backends implement.