fix(providers): use Bearer auth for Gemini CLI OAuth tokens

* fix(providers): use Bearer auth for Gemini CLI OAuth tokens

When credentials come from ~/.gemini/oauth_creds.json (Gemini CLI),
send them as Authorization: Bearer header instead of ?key= query
parameter. API keys from env vars or config continue using ?key=.

Fixes #194

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor(gemini): harden OAuth bearer auth flow and tests

* fix(gemini): granular auth source tracking and review fixes

Build on chumyin's auth model refactor with:
- Expand GeminiAuth to 4 variants (ExplicitKey/EnvGeminiKey/EnvGoogleKey/
  OAuthToken) so auth_source() uses stored discriminant without re-reading
  env vars at call time
- Add is_api_key()/credential() helpers on the enum
- Upgrade expired OAuth token log from debug to warn
- Add tests: provider_rejects_empty_key, auth_source_explicit_key,
  auth_source_none_without_credentials

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* style: apply rustfmt to fix CI lint failures

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: root <root@instance-20220913-1738.vcn09131738.oraclevcn.com>
Co-authored-by: argenis de la rosa <theonlyhennygod@gmail.com>

src/skillforge/mod.rs

@@ -78,10 +78,7 @@ impl std::fmt::Debug for SkillForgeConfig {
             .field("sources", &self.sources)
             .field("scan_interval_hours", &self.scan_interval_hours)
             .field("min_score", &self.min_score)
-            .field(
-                "github_token",
-                &self.github_token.as_ref().map(|_| "***"),
-            )
+            .field("github_token", &self.github_token.as_ref().map(|_| "***"))
             .field("output_dir", &self.output_dir)
             .finish()
     }
@@ -155,7 +152,10 @@ impl SkillForge {
                     }
                 }
                 ScoutSource::ClawHub | ScoutSource::HuggingFace => {
-                    info!(source = src.as_str(), "Source not yet implemented — skipping");
+                    info!(
+                        source = src.as_str(),
+                        "Source not yet implemented — skipping"
+                    );
                 }
             }
         }