feat(huddle): offer the team's server-side agents in the huddle picker

[mattbalza]

What

Add-agent in a huddle listed only the agents this desktop runs itself. A huddle started from a channel where @erp, @codex, @claude and @seek answer every day could not have one of them in it — even though the backend has accepted a pubkey and sent the kind:9000 all along (huddle/agents.rs::add_agent_to_huddle). The gap was entirely in the picker, which populated from list_managed_agents only.

The picker now shows the union of managed agents and the relay directory, scoped exactly like a mention: an agent appears only if the viewer could have @mentioned it in the huddle's parent channel (getMentionableAgentPubkeys with eligibilityScope: { type: "channel", channelId }). Allowlists and owner-only agents stay where they were.

Nothing auto-joins. The picker is the only way in.

Shape

• desktop/src/features/huddle/lib/huddleAgentCandidates.ts — new. huddleAllowsAgents(parentChannelType) and buildHuddleAgentCandidates(...). Managed agents win when an agent is in both sources; already-in-huddle pubkeys are dropped case-insensitively; with no parent channel id it returns managed only, because community scope would offer agents that cannot speak here.
• AddAgentDialog.tsx — takes parentChannelId, unions the two sources, tags each entry data-agent-source="managed" | "remote" and badges remote ones Team.
• Remote entries carry no lifecycle. Their process runs on the server under their own identity, so adding one is a membership event and nothing more: no start_managed_agent, no stop_managed_agent, no rollback aimed at a process this desktop does not own. The existing parent_error warning path is unchanged.
• HuddleBar.tsx — a huddle whose parent is a DM offers no agents at all, the same rule DM threads already follow. HuddleState carries the parent's id but not its type, so the type is resolved from the channel list.

Test plan

• node --import ./test-loader.mjs --experimental-strip-types --test "src/features/huddle/**/*.test.mjs" → 29 pass, 0 fail (9 new, covering the mention rule per channel type, per-viewer allowlists, dual-source dedupe, and the no-parent case).
• npx playwright test --project=smoke tests/e2e/huddle-agents.spec.ts → 3 passed: a parent-scoped relay agent is offered as remote; one scoped to another channel is not; clicking a remote entry logs add_agent_to_huddle and neither start_managed_agent nor stop_managed_agent; a DM-parented huddle renders no Add-agent button.
• tsc --noEmit and biome check clean.

Verified the pre-fix bundle renders only the managed agent with no data-agent-source — which is what these tests catch.

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