Add unit tests for the starter/runner core logic (leverage the unused memory datastore as a fake)

[whywaita]

Problem

The core of the system is untested. Of 63 non-test Go files, only 3 packages have tests (pkg/datastore/mysql, pkg/gh, pkg/web). pkg/starter (589 lines) and pkg/runner have zero tests, including the runner-deletion safety valve.

Highest-value, lowest-cost targets (pure functions)

• sanitizeRunner / sanitizeRunnerMustRunningTime / sanitizeGitHubRunner (pkg/runner/runner_delete.go:213-241) — these decide whether a runner may be deleted; a bug here can force-delete a runner with a running job.
• extractWorkflowIDs (pkg/starter/starter.go:193)
• ExtractRunsOnLabels (pkg/gh/webhook.go:35) — called from 4 sites.
• labelsToOneLine (pkg/starter/scripts.go:124), ToName/ToUUID/ToReason (pkg/runner/util.go), GetRunnerTemporaryMode (pkg/runner/runner.go:102).

Enabler: revive the unused memory datastore

pkg/datastore/memory/memory.go implements the Datastore interface but is referenced from nowhere (dead code). Using it as a fake would let Starter.ProcessJob (pkg/starter/starter.go:212) be tested without a MySQL container.

Existing test bug to fix along the way

pkg/web/target_test.go:578 uses if strings.EqualFold(test.want, got) — it fails on a match, i.e. the condition is inverted, so the error-response assertion is effectively disabled. Should be !strings.EqualFold(...).

[whywaita]

One more high-value target to add to the list: HandleGitHubEvent / receiveWorkflowJobWebhook (pkg/web/webhook.go) integration tests. It is the entry point of the whole system (signature validation → event dispatch → runs-on label filtering → job enqueue), and internal/testutils already boots the full web.NewMux on an httptest.Server with a real MySQL — so POSTing signed workflow_job payloads and asserting on the resulting jobs rows is cheap to write with the existing harness.