feat(google_adk): add ADK-112, workflow agent has no description

[bradAGI]

adk_sequential_agent, adk_parallel_agent, and adk_langgraph_agent are valid applies_to tokens the engine discovers but no shipped rule used any of them — every ADK agent-scope rule targets adk_llm_agent, except ADK-108 for LoopAgent. I found this auditing which engine capabilities have zero rule coverage.

This extends ADK-101's routing check to the workflow classes. A workflow agent is routed to exactly like an LlmAgent: when the parent decides whether to hand off, the model sees only the child's description, so one without a description sits in the sub_agents tree unreachable through delegation — a silent routing bug, not an error.

It's easier to miss on a workflow agent than on an LlmAgent, which is why it's worth its own rule rather than widening ADK-101. A workflow agent has no instruction= and no model= of its own, so nothing else about its construction reads as model-facing text and the missing description doesn't look like an omission. What it orchestrates is also invisible from outside — the parent can't infer "runs these three steps in order" from the class name. The fix leans on that: describe what the workflow accomplishes, not its mechanics, since the parent's model is choosing whether this branch is the right one.

adk_langgraph_agent is deliberately left out. LanggraphAgent takes a compiled graph rather than an ADK sub_agents tree, so the delegation-routing argument doesn't obviously transfer — I'd want to check how the engine models it before claiming it in user-facing text. Happy to follow up if it does apply.

Numbered ADK-112 to avoid colliding with ADK-111 in the open PR #51.

Verification — engine built at main:

$ trustabl rules validate .
OK: 85 rule pack(s), 207 rule(s) valid under rule schema version 14

Fire (SequentialAgent, ParallelAgent, and LoopAgent each with no description=): ADK-104, ADK-112, ADK-201 — three separate ADK-112 findings, one per class, so all three applies_to values are exercised rather than assumed.
Silent (all three with a description=): ADK-104, ADK-201

(ADK-104 and ADK-201 are pre-existing rules firing on the fixture's LlmAgents and missing AGENTS.md.)

No new predicates, so no schema_version bump.