fix: update Gemini model to 2.5-flash and add API auto-detection in enhance

Fix 1 — gemini.py: replace deprecated gemini-2.0-flash-exp (404 errors)
with gemini-2.5-flash (stable, GA, Google's recommended replacement).
Closes #290.

Fix 2 — enhance dispatcher: implement the documented auto-detection that
was missing from the code. skill-seekers enhance now correctly routes:
  - ANTHROPIC_API_KEY set → Claude API mode (enhance_skill.py)
  - GOOGLE_API_KEY set    → Gemini API mode
  - OPENAI_API_KEY set    → OpenAI API mode
  - No API keys           → LOCAL mode (Claude Code Max, free)

Use --mode LOCAL to force local mode even when an API key is present.

9 new tests cover _detect_api_target() priority logic and main()
routing (API delegation, --mode LOCAL override, no-key fallback).

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

src/skill_seekers/cli/adaptors/gemini.py

@@ -3,7 +3,7 @@
 Google Gemini Adaptor
 
 Implements platform-specific handling for Google Gemini skills.
-Uses Gemini Files API for grounding and Gemini 2.0 Flash for enhancement.
+Uses Gemini Files API for grounding and Gemini 2.5 Flash for enhancement.
 """
 
 import json
@@ -23,7 +23,7 @@ class GeminiAdaptor(SkillAdaptor):
     - Plain markdown format (no YAML frontmatter)
     - tar.gz packaging for Gemini Files API
     - Upload to Google AI Studio / Files API
-    - AI enhancement using Gemini 2.0 Flash
+    - AI enhancement using Gemini 2.5 Flash
     """
 
     PLATFORM = "gemini"
@@ -279,7 +279,7 @@ See the references directory for complete documentation with examples and best p
 
     def supports_enhancement(self) -> bool:
         """
-        Gemini supports AI enhancement via Gemini 2.0 Flash.
+        Gemini supports AI enhancement via Gemini 2.5 Flash.
 
         Returns:
             True
@@ -288,7 +288,7 @@ See the references directory for complete documentation with examples and best p
 
     def enhance(self, skill_dir: Path, api_key: str) -> bool:
         """
-        Enhance SKILL.md using Gemini 2.0 Flash API.
+        Enhance SKILL.md using Gemini 2.5 Flash API.
 
         Args:
             skill_dir: Path to skill directory
@@ -338,7 +338,7 @@ See the references directory for complete documentation with examples and best p
         try:
             genai.configure(api_key=api_key)
 
-            model = genai.GenerativeModel("gemini-2.0-flash-exp")
+            model = genai.GenerativeModel("gemini-2.5-flash")
 
             response = model.generate_content(prompt)