fix(runners): InMemoryRunnerRegistry has no unregister/deregister method

[OneStepAt4time]

Problem

InMemoryRunnerRegistry (src/runners/registry.ts) has register() but no unregister() method. When #3180 adds dynamic runner registration (hot-pluggable agents), there is no way to remove a runner at runtime.

Current behavior

• First registered runner becomes defaultRunner — permanently
• No way to deregister a crashed/unavailable runner
• If the default runner goes down, getDefault() returns a dead reference

Missing capabilities

1. unregister(name: string): boolean — remove a runner
2. setDefault(name: string): void — explicit default override
3. has(name: string): boolean — existence check without get()

Impact

• P3 now, but will block feat: multi-agent support — strategic gap exposed by OpenACP (28+ agents) and cc-connect (10+ agents) #3180 implementation
• Dynamic runner lifecycle management requires add AND remove

Discovered during

Code quality sweep on src/runners/ (pre-#3180 audit).