Add @oagen-ignore-file to hand-written files

Marks 27 files that are entirely hand-written so oagen never
overwrites them during generation. Covers the main client,
entry points, crypto providers, HTTP transport, utilities,
and domain-specific classes (Webhooks, Actions, Vault, etc.).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>