ci: run independent checks in parallel

[matanyall]

Run lint and tests concurrently after typechecking finishes. Keep typechecking and the subsequent build serial because both regenerate the API client.

Validation: parsed the workflow YAML and compared flattened steps against latest main; every command, condition, permission, and required-check name is preserved. Whitespace validation passed. Remote CI has passed at the current head.

References: GitHub parallel steps, Depot parallel steps.

Remote verification: 7441bb4137. All Node 20, 22, and 24 legs passed. In every leg, lint and tests have identical start timestamps; each parallel group joined successfully before the build.

[polylane]

Note

Production impact unlikely.

Documentation, test, or CI-only change; nothing that runs in production is affected.

_{Polylane analysed 7441bb4 for production impact. You can ask follow-ups by mentioning @polylane in a comment.}

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[matanyall]

Verified at head 7441bb413724874ea33b53ac3d4200df249890bb.

Remote CI run: All Node 20, 22, and 24 legs passed. In every leg, lint and tests have identical start timestamps; each parallel group joined successfully before the build.

Workflow YAML and flattened steps were compared against main: all commands, conditions, permissions, and required-check names are preserved.

Switchboard review was requested but its harness failed with HTTP 500: the original session policy checkpoint was not acknowledged. No head-pinned Switchboard review has been posted.

Leaving this PR open for human review and merge.