UI: @ autocomplete to reference other running sessions / agents

[heavygee]

Problem

Operator wants to cross-reference one running agent from another -- "this agent should consult @voice-dropdown about the rebase" or "ping @server-setup to add this path to backups." Currently this is done by pasting raw session UUIDs which the agent has to manually resolve.

Proposal

Add @ autocomplete in the message composer:

1. Typing @ triggers an autocomplete popup
2. Popup shows running sessions (with tags / project / last activity), most-recently-active first
3. Selecting one inserts a structured reference: @session:<id> (or @<tag> if the session is tagged)
4. The reference renders as a clickable chip in the message bubble
5. Receiving agent's tool catalog includes a lookupSession({sessionId}) tool (or a sendToSession({sessionId, message}) tool) so the agent can either read context or pass a message

Analogy: Discord @user mentions. Cross-agent message passing.

Acceptance

• @ triggers autocomplete in the composer
• Selecting inserts a structured reference token
• Reference renders as a chip with hover-card (mini summary like Hub: background session notes without CLI dispatch + web normalize fix #4)
• Receiving agent can resolve the reference and either fetch context or push a message
• Permission model: by default, the source agent can READ the target session's recent context but only the operator can send-as

Notes

• Pair with Hub: background session notes without CLI dispatch + web normalize fix #4 (hover popup uses the same lookup).
• Pair with Tooling: agent-driven fileFutureIdea tool that posts to fork issues #10 (project page may surface inter-session references as a graph).
• This is the primary mechanism for the agent network to become aware of itself.