Prefer native delivery over legacy DING

[myobie]

An agent can keep an obsolete ding entry after it moves to native delivery. st2 rejected the whole declaration, which made unrelated catalog reads fail.

This change lets native delivery win. Agent Spec lowering omits the DING task whenever a driver, session owner, readiness contract, or native transport exists.

Tests cover typed drivers, session drivers, readiness, and legacy deliver declarations.

Verification

• cargo test -p agent-spec
• cargo test --test driver_expansion

[schickling-assistant]

Evidence from dev3 (2026-09-06), for your call on this PR:

• 0 of 94 live declarations and 0 of 668 archived declarations carry a ding node (rg '^\s*ding' $CATALOG/agents/dev3/*/agent.kdl), and the dotfiles catalog renderer no longer emits one for native-harness seats.
• main already refuses ding + native delivery at admission, so the failure this PR tolerates ("an obsolete ding entry made unrelated catalog reads fail") cannot currently occur on the fleet.
• Per today's decisions with Johannes (seat dev3.direct.omp.43sz6ujq, Q1 incidents-first / Q19 lifecycle), the orchestration lane is not merging or closing contributor PRs; this is your PR to close or keep. If you keep it, the alternative that removes a field rather than tolerating it would be: keep the refusal, and delete ding from any remaining emitter.

Posted on behalf of @schickling

field	value
agent_identity	dev3.direct.omp.43sz6ujq
session	dev3.43sz6ujq
agent_persona	generalist
agent_supervisor	unavailable
agent_tool	OMP
agent_tool_version	18.1.7
agent_runtime	OMP 18.1.7
tooling_profile	dotfiles@39a19af