refactor: use shared extheartbeat instead of local heartbeat package

[joshiste]

What

Switches preflight_kit_sdk to the shared extheartbeat watchdog from extension-kit (extension-kit#150, released v1.10.7) and deletes the local preflight_kit_sdk/v2/heartbeat package.

It was a byte-identical copy of action-kit's heartbeat monitor — the same concurrency bug had to be fixed in both (preflight-kit#52, action-kit#457). Sharing one implementation stops it diverging again. extension-kit was already a dependency, so no new module coupling. Companion to action-kit#458.

The re-Start monitor-leak fix from #52 (Swap-and-Stop the previous monitor) is preserved.

API impact

None. The local heartbeat package was SDK-internal (imported only by preflight_sdk.go, not by extensions). Clean removal, agreed.

Verification

• extension-kit bumped to v1.10.7; local heartbeat package removed.
• go build ./..., go vet ./..., go test -race ./... pass (module go/preflight_kit_sdk); existing stop/heartbeat/leak tests still pass.

[claude]

Claude finished @joshiste's task in 2m 54s —— View job

---

Claude's Task List

• Check for previously unresolved review threads from Claude on this PR (none found)
• Gather context (diff, changed files)
• Review new changes for bugs, security, concurrency, performance
• Post final review feedback

Review

This is a clean, mechanical refactor — no functional/behavioral concerns.

• go/preflight_kit_sdk/preflight_sdk.go: heartbeat.Notify/heartbeat.Monitor calls are swapped 1:1 for extheartbeat.Notify/extheartbeat.Monitor in monitorHeartbeatWithCallback, recordHeartbeat, and stopMonitorHeartbeat. The Swap-and-Stop logic from fix: prevent data races and panics in SDK stop/heartbeat handling #52 (preventing monitor leaks on repeated Start) and the LoadAndDelete-based idempotent stop are preserved verbatim.
• The local heartbeat package (and its test file) is fully removed; git grep confirms no remaining references to preflight_kit_sdk/v2/heartbeat anywhere in the repo.
• go.mod/go.sum are bumped consistently to extension-kit v1.10.7 (hash and go.mod hash both updated).
• CHANGELOG.md entry accurately describes the change.
• No API surface change, matching the PR description — the old package was SDK-internal.
• CI (ci workflow: build/vet/test) passed on this commit, confirming the swap compiles and existing heartbeat/stop/leak tests still pass.

No bugs, security issues, or concurrency issues found in the diff itself.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud