v0.4.1 — sequence diagrams + self-contained flow authoring

flows v1 finished for every harness: a sequence diagram projection (flow_render: sequence — same flow-meta, actors→participants, edges→messages; verification unchanged), and the flow authoring loop + verified-vs-inferred rule folded into the vendored AGENTS.md so a target repo can draft a flow without the plugin-side references/flow.md (it becomes the optional deep reference).