feat(import-export): complete company portability — secrets export/import and env round-tripping

Adds opt-in secret export/import: secret values are resolved (and optionally
decrypted) into the portability manifest, and re-created with conflict
handling on import. Fixes env round-tripping so both secret_ref and plain
bindings survive export/import cycles.

packages/shared/src/index.ts

@@ -473,6 +473,7 @@ export type {
   CompanyPortabilityImportRequest,
   CompanyPortabilityImportResult,
   CompanyPortabilityExportRequest,
+  CompanyPortabilitySecretEntry,
   EnvBinding,
   AgentEnvConfig,
   CompanySecret,