[11.0][A3] Remove legacy environment-variable aliases (OS_MULTI_TENANT, …)

[os-zhuang]

Part of 11.0 breaking-change batch (#2364).

Remove the legacy environment-variable aliases. (Owner decision: remove in 11.0, not deferred to v12.)

Scope

• packages/types/src/env.ts — readEnvWithDeprecation + the legacy fallbacks (e.g. OS_MULTI_TENANT → OS_MULTI_ORG_ENABLED; review PORT → OS_PORT).
• Consumers: resolveMultiOrgEnabled() used by packages/cli/src/commands/serve.ts and packages/plugins/plugin-dev.

Why this cascades

This is a runtime/deployment-config break, not a code API. Any deployment still starting with the old env names will read no value after removal.

Checklist

• Drop legacy-alias handling; read canonical names only
• Update callers (serve.ts, dev-plugin) + any example/CI env
• UPGRADE.md: document the renamed env vars (old → new)
• changeset

Risk: low (code), but ensure docs/examples/CI all use the new names + a clear upgrade note.