feat(agent-kits): add built-in agent kit metadata and inference #479

Intent

Implement the first acq-side increment from ADR-0030: make agent kits first-class enough that acq run <agent> <workspace> can resolve an agent kit by built-in-bundle name and attach through that kit's declared entrypoint.

Parent: #473
ADR: #474

Scope

• Define the minimal agent-kit metadata needed for create/run: agent name, install/start ownership, and entrypoint command.
• Resolve implicit agent kits from the built-in bundle only.
• Never resolve implicit agent kits from ACQ_EXTRA_KITS.
• Print the selected agent kit during create/run, or expose equivalent inspection via acq kit ls.
• Preserve shell as the no-agent-kit case.
• Keep explicit --kit behavior higher precedence than inference.

Deferred Decisions

• Exact merge semantics for non-egress kit conflicts remain deferred until a later increment needs them.
• Provider config rendering remains out of scope for this increment.
• Moving OpenCode itself into a kit is tracked separately.

Acceptance Criteria

• Existing acq run opencode . behavior remains stable from the user's perspective.
• A built-in agent kit can be selected implicitly and visibly.
• A same-named team or personal kit cannot silently become the selected agent kit.
• Tests cover built-in inference, extra-kit non-inference, explicit override, and shell.

Orchestration Notes

Implementation should be done by an authoring sub-agent and reviewed by an independent adversarial review sub-agent. Review findings must be addressed before the increment is accepted.