## Why
`close_agent` is the wrong model-facing name for the v2 operation after
the residency changes. V2 agents remain reusable by task name, and
residency/unloading owns capacity management; the exposed tool should
describe the action it actually performs: interrupt the target agent's
current turn without making the agent unavailable for future messages or
follow-up tasks.
## What changed
- Rename the multi-agent v2 tool from `close_agent` to
`interrupt_agent`.
- Keep the v1 `close_agent` surface unchanged.
- Update the v2 handler to send `Op::Interrupt`, keep interrupted agents
registered, and reject root/self targets with interrupt-specific errors.
- Route interrupt delivery through the existing dead-thread cleanup path
so stale resident entries do not keep consuming capacity.
- Update tool planning and handler tests for the new v2 surface and
semantics.
## Verification
Added focused coverage in:
- `core/src/tools/spec_plan_tests.rs`
- `core/src/tools/handlers/multi_agents_tests.rs`
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