feat: relaunch runseal beta foundation

[PerishCode]

v0.1.0-beta.0

• Fresh-repo cold start for runseal, with the product line still intentionally thin.
• The current floor is explicit env injection plus stable symlinked entrypoints, not a broad workflow platform.
• Helpers stay explicit, with :node now able to self-bootstrap a managed node + npm baseline.
• pnpm and yarn already absorb into the same managed Node version root through normal runtime actions.
• corepack is partially integrated: managed cache and shim surfaces exist, but the user-facing semantics are not final yet.

Beta boundary

• This beta proves the runtime shape and helper boundary.
• It does not freeze the long-term :node or corepack semantics.