Rename encryption client config.strategy to authStrategy; document auth + keysets

[coderdan]

Summary

Rename the encryption client's auth strategy field from strategy to authStrategy to make its purpose clear, and document the client's authentication and keyset model properly in the Encryption() TypeDoc.

Scope

• Rename (backwards compatible): ClientConfig.authStrategy becomes the documented field; ClientConfig.strategy is retained as a @deprecated alias that still works but logs a one-time runtime deprecation warning. authStrategy wins when both are set.
• TypeDoc rewrite for Encryption():
  • Default auto strategy (env vars → local dev profile, which also supplies the client key).
  • npx stash auth login for local development.
  • The four CS_* env vars for production/CI (table + dashboard link).
  • Custom strategies via authStrategy — AccessKeyStrategy and OidcFederationStrategy.
  • Lock context as an identity-bound capability on top of OidcFederationStrategy.
  • Keysets for multi-tenant isolation (one client per tenant).
• Tests for the authStrategy field, the deprecated strategy alias (forwarding + warning), and precedence.

Resolved by

#562

Related follow-ups

• Allow passing a keyset to encrypt/decrypt operations, not just at init #561 — per-operation keyset (encrypt/decrypt), not just at init.
• Document the isolation hierarchy: keysets vs auth strategies vs lock context docs#47 — document the isolation hierarchy (keysets vs auth strategies vs lock context).
• Document creating keysets programmatically (and link to stack TypeDoc) docs#48 — document creating keysets programmatically.