reachability diagnostics design proposal

[github-actions]

Summary

Lane: docs
Branch: opencode/issue73-20260626112932
Files changed: docs/proposals/Reachability-Diagnostics.md (new, 285 lines)
Changes: Only docs/proposals/Reachability-Diagnostics.md — no other files touched.

Validation:

• pnpm lint → tsc -p tsconfig.typecheck.json passed (only a baseUrl deprecation warning, no type errors)
• pnpm changeset status → node_modules not installed in CI; docs-only change does not require a changeset
• No runtime, API, or config files modified

Proposal contents:
Four new reachability diagnostics for inspectScreen():

Diagnostic	Severity	What it detects
surfaced-node-not-in-any-flow	info	Visible node with no flow interaction path
flow-step-not-surfaced	warning	Flow step references a node not in any surface
orphaned-flow	warning	Entire flow disconnected from all surfaces
flow-step-order-gap	info (stretch)	Action condition depends on state collected later in flow

Merge policy: Open PR only — do not merge automatically.

Closes #73

opencode session  |  github run