fix(agent): resolve oauth cache home cross-platform (#5151)

## Summary

- resolve the OAuth cache home directory with the repository's
platform-aware `dirs` convention
- preserve the existing `.config/buzz-agent/oauth` cache layout on macOS
and Linux
- make the cache-path regression assertion portable across path
separators

## Problem

`buzz-agent` read only `$HOME` when constructing the OAuth token cache
path. Packaged Windows processes do not guarantee that Unix variable, so
OAuth source construction failed with `oauth cache: $HOME not set` even
though Windows had a valid user profile.

## Validation

Independent reviewers validated exact commit
`836d820483b141b7291170cb33535ac7cb49b2eb` on Windows/MSVC with `HOME`
unset and `USERPROFILE` present:

- `cargo +1.94.1 clippy -p buzz-agent --all-targets --locked -- -D
warnings`
- `cargo +1.94.1 fmt --all -- --check`
- `git diff --check f53bbd1..836d820`
- `auth::` tests: 11/11 passed with `HOME` unset
- full package lib target: 396 passed / 2 failed; identical-base
controls classified both as pre-existing Windows failures

The changed regression fails on the base with `$HOME not set` and passes
on this branch. `dirs 6.0.0` was already locked by other workspace
crates; the lockfile change adds only the `buzz-agent` dependency edge.

Signed-off-by: Kalvin C <kalvinnchau@users.noreply.github.com>

Cargo.lock

@@ -862,6 +862,7 @@ dependencies = [
  "async-trait",
  "axum",
  "base64 0.22.1",
+ "dirs",
  "getrandom 0.4.3",
  "hex",
  "nix 0.31.3",

crates/buzz-agent/Cargo.toml

@@ -43,6 +43,7 @@ hex = { workspace = true }
 sha2 = { workspace = true }
 urlencoding = "2"
 webbrowser = "1"
+dirs = "6"
 
 [target.'cfg(unix)'.dependencies]
 nix = { version = "0.31", default-features = false, features = ["signal", "process"] }

crates/buzz-agent/src/auth.rs

@@ -453,15 +453,12 @@ fn cache_path_for(cfg: &PkceOAuthConfig) -> Result<PathBuf, AgentError> {
 
     let dir = match &cfg.cache_dir_override {
         Some(p) => p.join(&cfg.cache_namespace),
-        None => {
-            let home = std::env::var("HOME")
-                .map_err(|_| AgentError::Llm("oauth cache: $HOME not set".into()))?;
-            PathBuf::from(home)
-                .join(".config")
-                .join("buzz-agent")
-                .join("oauth")
-                .join(&cfg.cache_namespace)
-        }
+        None => dirs::home_dir()
+            .ok_or_else(|| AgentError::Llm("oauth cache: home directory not found".into()))?
+            .join(".config")
+            .join("buzz-agent")
+            .join("oauth")
+            .join(&cfg.cache_namespace),
     };
     Ok(dir.join(format!("{hash}.json")))
 }
@@ -683,8 +680,7 @@ mod tests {
     }
 
     #[test]
-    fn cache_path_includes_namespace_and_hash() {
-        // HOME is required; cargo test runs set it.
+    fn cache_path_uses_platform_home_directory() {
         let cfg = PkceOAuthConfig {
             discovery_url: "https://example.com/.well-known".into(),
             client_id: "abc".into(),
@@ -693,8 +689,14 @@ mod tests {
             cache_dir_override: None,
         };
         let p = cache_path_for(&cfg).unwrap();
-        assert!(p.to_string_lossy().contains("/buzz-agent/oauth/demo/"));
-        assert!(p.extension().and_then(|s| s.to_str()) == Some("json"));
+        let expected_dir = dirs::home_dir()
+            .unwrap()
+            .join(".config")
+            .join("buzz-agent")
+            .join("oauth")
+            .join("demo");
+        assert_eq!(p.parent(), Some(expected_dir.as_path()));
+        assert_eq!(p.extension().and_then(|s| s.to_str()), Some("json"));
     }
 
     #[test]