(v1) CLI scaffold: collapse to canonical env-object family + v0→v1 migration

[yamcodes]

Summary

Collapse the CLI scaffold to one canonical family: every framework scaffolds env.ts as the imported env object surface. The #1316 codegen-vs-plugin-env seam dissolves per ADR 0015.

Product call (#1333): Option 3. Schema/define is not a scaffold variant, escape hatch, or soft-deprecated path on v1 — omit it entirely. Teach and scaffold import { env } from "./env" only. Provide a migration path that detects v0 schema/define patterns and rewrites them to the canonical env-object surface when users upgrade.

Scope

• Vite/Bun default scaffold: env.ts (createEnv) + plugin registration wired for the transform design ((v1) feat(vite-plugin): env-module transform — canonical env object surface #1328 / (v1) feat(bun-plugin): env-module transform — canonical env object surface #1329) — no vite-env.d.ts / bun-env.d.ts injection.
• No schema/define scaffold variant; no planDtsFile SPA path.
• Fold isCodegenFramework / createCodegenFrameworkStrategy vs plugin-env strategy split into the single family; clientPrefix first-class for all frameworks (ADR 0014 consequence, (v1) Add clientPrefix to all FrameworkStrategy implementations #1320).
• Hosting presets (Add hosting presets to init CLI #1288) emit into the unified env.ts shape for all frameworks.
• Migration: detect v0 schema/define layouts and automatically migrate to import { env } from "./env" / canonical env-object layout.

Ownership boundary

This issue owns CLI scaffold + migration UX only.

Package API removal of schema/define on v1 is #1461 (separate ready-for-agent chore). Coordinate sequencing so migration UX matches the surface packages actually ship.

Acceptance criteria

• arkenv init on a Vite or Bun project scaffolds the env-object surface by default and compiles/runs.
• No schema/define / .d.ts scaffold variant exists; absence is intentional (Option 3).
• Next/Nuxt scaffolds unchanged in outcome (already env-object).
• Scaffold strategy code has one framework family, not two.
• Migration detects v0 schema/define patterns and rewrites to the canonical env-object surface.

Landing

v1 branch only — see ADR 0015 landing strategy (as amended by #1333). Depends on #1328 / #1329. Coordinates with #1289 / #1311 / #1316 / #1320 / #1461. Part of #1105 (ADR: #1326). Unblocked by #1333 Option 3.

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