v0.1.2 - OpenAI OAuth and agent plugins

Compared against v0.1.1 (a52c061).

Published crate bumps:

• halter-config 0.1.1 -> 0.1.2
• halter-providers 0.1.1 -> 0.1.2
• halter 0.1.1 -> 0.1.2

Changes:

• Adds Halter Rust agent plugin manifests and marketplace entries, including basic-rust and workflows-rust skills.
• Adds halter auth openai-oauth for obtaining OpenAI OAuth token bundles and rendering env/JSON output.
• Adds [providers.openai].oauth config support and provider wiring for OpenAI OAuth credentials.
• Routes OAuth OpenAI traffic through the ChatGPT Codex backend with top-level instructions and store: false where required.
• Fixes OAuth request encoding, bearer credential handling, and OpenAI/OpenRouter authentication error wording.