RFC: Flows as .flow.jsonc — composable flow-graph format

[apotema]

Umbrella tracking issue for the Flows-as-.flow.jsonc RFC.

Summary

Move flow graphs from .flow.zon to .flow.jsonc, flatten the node schema, and make subgraph composition work through named registry references — reusing the RFC #560 registry / resolver / #569 cycle-detector machinery.

• Format — .flow.jsonc (JSONC), replacing .flow.zon.
• Schema — flat nodes + edges; per-node tagged-union nesting dropped.
• Composition — a Subflow node references another flow by registry name; the referenced flow declares a named params interface; overrides binds those parameters by name (Model B). Reference cycles caught at load time.

The full RFC text is RFC-FLOWS-JSONC.md, added by the PR linked below.

Sub-tickets

• flow-codegen: parse .flow.jsonc + flat schema + Subflow/params codegen #1 — parse .flow.jsonc + flat schema + Subflow/params codegen
• Flows editor: .flow.jsonc read/write + Subflow & Param nodes labelle-gui#153 — Flows editor: .flow.jsonc read/write + Subflow & Param nodes
• Flow discovery: scan scripts/flows/**/*.flow.jsonc labelle-assembler#170 — flow discovery scans *.flow.jsonc
• Converter: .flow.zon → .flow.jsonc + migrate in-tree files #2 — .flow.zon→.flow.jsonc converter + migrate in-tree files

Open questions (non-blocking)

1. Pin projection across a Subflow reference boundary (ties into labelle-gui#44).
2. Editor round-trip stability (deterministic .flow.jsonc re-save).

🤖 Filed via automated triage