chore: justfile maintenance

[adamXbot]

Adds a root justfile per the org convention (recipes delegate to the existing CI/release commands). License untouched — already correct.

just --list output:

Available recipes:
    default         # List available commands

    [dev]
    build           # Build every SPM target (Core, auditctlKit, auditctl, guest agent, protocol)
    clean           # Remove SPM build output
    test            # Run the full SPM test suite (same as CI)

    [ship]
    release version # Tag v<version> and push it to trigger the release workflow (tag must match MARKETING_VERSION)
    release-local   # Build, sign, notarize and package the DMG locally (needs Developer ID + notary env)

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