agent: document multicast device capacity and add capacity eval

[armcconnell]

Resolves: #651

Summary of Changes

• Add a "Device multicast capacity" section to the agent's SQL_CONTEXT.md so it can answer questions like "which DZDs have capacity for new subscribers". Previously the agent had no definition of device capacity and would reason over the stale on-chain multicast_subscribers_count / multicast_publishers_count columns on dz_devices_current, producing nonsensical answers.
• The new guidance: marks the on-chain device count columns as unreliable; defines current subscribers/publishers as activated multicast users on the device with a non-empty subscribers/publishers array (from dz_users_current); defines available capacity as max_multicast_subscribers - current, only meaningful when max_multicast_subscribers > 0 (0 means unknown, not full); and includes a canonical "which devices have capacity" query.
• Add the MulticastDeviceCapacity eval. It seeds devices whose stale on-chain counts deliberately disagree with their attached users (a: on-chain 0 vs live 2; c: on-chain 8 vs live 0), so the eval fails if the agent reads the device count columns instead of counting live users.

This is the agent-facing half of the multicast count problem. The data/display half is #650 (PR #653).

Diff Breakdown

Category	Files	Lines (+/-)	Net
Docs (prompt)	1	+~35 / -1	+~34
Tests (eval)	1	+~185 / -0	+~185

Prompt guidance plus a new eval; no production code paths change.

Testing Verification

• MulticastDeviceCapacity eval passes against the live agent (claude-haiku-4-5): the agent generated a query counting live subscribers via countIf(JSONLength(subscribers) > 0) and computing max - current, reported nyc-dzd-c (10 available) and nyc-dzd-a (3 available), correctly excluded the full nyc-dzd-b, and used live counts (a=2, c=0) rather than the stale on-chain values (a=0, c=8). The eval's LLM judge passed all three expectations.
• The eval's ground-truth ClickHouse query is independently asserted (a=3, b=0, c=10 available) so the scenario is verified even in short mode.