feat(comms): wire ReparentChannel and attached CreateChannel (RIG-4190)

[rigel-mintaka]

This PR is part of a stack containing 5 PRs:

1. main
2. feat(store): attach channels to agents with tree or explicit membership (RIG-4186) #1762
3. feat(store): authorize channel writes and reads by tree participation (RIG-4187) #1764
4. feat(store): owner-set visibility and derived TREE members on channel reads (RIG-4188) #1772
5. feat(store): TREE subscriptions and delivery over derived participants (RIG-4189) #1778
6. "feat(comms): wire ReparentChannel and attached CreateChannel (RIG-4190)" (this PR)

Summary

Channels-in-the-agent-tree, task T6 (docs/designs/ui/compass-channels-in-agent-tree/design.md § Plan): the RPC edge and events. Stacked on #1778.

• CreateChannel passes parent_agent_handle and membership_mode to the store. An unknown mode is INVALID_ARGUMENT, so the call does not quietly create an EXPLICIT channel.
• ReparentChannel replaces the Unimplemented stub. It calls store.ReparentChannel and emits one ChannelChanged after commit. The stream's existing ChannelVisibleTo filter decides who receives it. When a channel leaves its anchor owner's tree (a detach, or a move to another owner's agent), the old owner set loses visibility. store.OwnerSetLostChannelVisibility finds those accounts in one query, and they are named in removed_account_ids so they still receive the final event. store.ReparentChannel now also returns the parent it replaced, read under its row lock.
• Stream: SubscribeComms trims removed_account_ids to the recipient's own id, so the list never shows one account another's id. This also covers the existing UpdateChannelMembers removals.
• Proto docs: the ChannelChanged comments list every emit cause and the per-recipient trim. Comment-only change; gen is regenerated.
• Handle resolution: a new resolveSameOwnerAgent (in resolve.go) resolves an optional agent handle. A foreign handle gets the same NOT_FOUND as an unknown one, naming the submitted handle. ReparentAgent's parent check now uses the same helper.
• No change to the coordination hook.

This also lands the RPC half deferred from #1772: the CreateChannel response and its ChannelChanged carry parent_agent_id and TREE mode.

Tests

• TestCreateChannelUnderAgentEmitsAttachedChannel: the response and exactly one ChannelChanged carry the anchor and TREE mode; a subtree agent's post succeeds end to end; a non-participant's post is NOT_FOUND.
• TestCreateChannelRejectsUnknownMembershipModeAndForeignAnchor: an unknown mode is INVALID_ARGUMENT; a foreign anchor is NOT_FOUND naming the submitted handle.
• TestReparentChannelEmitsOneChannelChangedToAdmittedAccounts: exactly one bus event; the owner's stream receives it; an outsider's stream receives its own later marker event first.
• TestReparentChannelDetachNotifiesViewersWhoLoseVisibility: a same-owner agent with no member row receives the detach event naming it as removed; the remaining member is not named.
• TestReparentChannelCrossOwnerMoveNotifiesOnlyLostViewers: a shared channel moved from X's agent to Y's agent; X's non-member agent receives the event naming only itself; the Y member receives it with no removed ids.
• TestOwnerSetLostChannelVisibilityExcludesRemainingPaths (store): nothing is lost while anchored in the owner set; after a detach only the non-member bystander is lost.
• TestReparentChannelNonParticipantIsNotFound: a non-participant gets NOT_FOUND; a foreign and an unknown destination give the same NOT_FOUND naming the submitted handle.
• Mutations, each failing a test above: publish removed; owner check removed (also fails the existing ReparentAgent foreign-parent test); mode dropped; double publish on reparent and on create; removed ids dropped on detach; plain handle resolve on the destination; no stream trim; detach-only notice; owner-set arm removed from the lost-visibility query.
• comms and auth suites (pgtest and unit): ok. Lint: clean.

Spec-impact: none
Ledger-impact: none

Refs RIG-4190, RIG-1622

Co-authored-by: Matt Wilkinson matt@rigel.build

[linear-code]

RIG-4190

RIG-1622

[github-actions]

Compass engineering docs preview: https://compass-comms-rig-4190-chann.compass-eng-docs.pages.dev

Deployed from compass-comms/rig-4190-channel-tree-rpc at 912d0fc.

[mattwilkinsonn]

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