Add arc-openai-oauth crate for OpenAI OAuth PKCE token acquisition

Standalone library implementing OpenAI's OAuth PKCE flow (browser + device)
for authenticating with ChatGPT subscriptions instead of API keys.

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

Cargo.lock

@@ -351,6 +351,24 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "arc-openai-oauth"
+version = "0.1.0"
+dependencies = [
+ "axum",
+ "base64",
+ "hex",
+ "mockito",
+ "open",
+ "rand 0.8.5",
+ "reqwest 0.12.28",
+ "serde",
+ "serde_json",
+ "sha2",
+ "tokio",
+ "tracing",
+]
+
 [[package]]
 name = "arc-slack"
 version = "0.1.0"

lib/crates/arc-openai-oauth/Cargo.toml

@@ -0,0 +1,26 @@
+[package]
+name = "arc-openai-oauth"
+edition.workspace = true
+version.workspace = true
+license.workspace = true
+description = "OpenAI OAuth PKCE token acquisition for Arc"
+
+[lib]
+doctest = false
+
+[dependencies]
+serde.workspace = true
+serde_json.workspace = true
+reqwest.workspace = true
+sha2.workspace = true
+base64.workspace = true
+rand.workspace = true
+hex.workspace = true
+tokio.workspace = true
+tracing.workspace = true
+axum = "0.8"
+open = "5"
+
+[dev-dependencies]
+mockito = "1"
+tokio = { workspace = true, features = ["test-util", "macros"] }