feat(ato-cli): best-effort Windows root taskkill for import preview

[Koh0920]

Summary

Part 6 of #306. This PR is intentionally narrow: it adds a best-effort Windows root taskkill path for import preview session stop.

What this PR does:

• when the recorded import preview root ato_run_pid is alive and its start time still matches, Windows stop uses taskkill /PID <root> /T to terminate that root tree
• keep-alive stop can use that Windows root-tree termination path
• Unix behavior is unchanged

What this PR does not do yet:

• it does not provide full Windows parity with the Unix orphan cleanup paths
• it does not stop verified child trees after the root process is already gone
• it does not sweep unrelated Windows processes based only on ATO_RUN_SESSION_ID

Validation note

This remains a draft until Windows runtime smoke is collected. Minimum validation still needed:

1. ato import --run --keep-alive --emit-json starts a child server on Windows
2. ato stop <run_session_id> removes the live root + child tree
3. root-dead child-orphan cleanup is still out of scope for this PR and should not be implied by this title/body

Out of scope

• full Windows process-tree parity for root-dead child cleanup
• new Unix changes

Tests

• macOS compile-path verification only
• Windows runtime smoke still required before merge

Part of #306.

[Koh0920]

Narrowed the PR scope/title/body to match the current implementation: this is now explicitly a best-effort Windows root taskkill /T change, not full Windows parity. The PR stays draft pending real Windows smoke before merge.

[Koh0920]

Keeping this PR in draft. The remaining gate is real Windows runtime smoke; the repository currently has no existing workflow that performs this runtime validation, and the available CI here is limited to Linux/macOS plus Windows packaging.

Remaining smoke on a Windows host:

1. ato import --run --keep-alive --emit-json
2. confirm the child server is running
3. ato stop <run_session_id>
4. confirm the live root + child tree are gone
5. keep root-dead child cleanup explicitly out of scope for this PR

Once those steps pass, this PR can move from draft to ready for review.