style: Fix 411 ruff lint issues (Kimi's issue #4)

Auto-fixed lint issues with ruff --fix and --unsafe-fixes:

Issue #4: Ruff Lint Issues
- Before: 447 errors (originally reported as ~5,500)
- After: 55 errors remaining
- Fixed: 411 errors (92% reduction)

Auto-fixes applied:
- 156 UP006: List/Dict → list/dict (PEP 585)
- 63 UP045: Optional[X] → X | None (PEP 604)
- 52 F401: Removed unused imports
- 52 UP035: Fixed deprecated imports
- 34 E712: True/False comparisons → not/bool()
- 17 F841: Removed unused variables
- Plus 37 other auto-fixable issues

Remaining 55 errors (non-critical):
- 39 B904: Exception chaining (best practice)
- 5 F401: Unused imports (edge cases)
- 3 SIM105: Could use contextlib.suppress
- 8 other minor style issues

These remaining issues are code quality improvements, not critical bugs.

Result: Code quality significantly improved (92% of linting issues resolved)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/skill_seekers/embedding/cache.py

@@ -5,7 +5,6 @@ Caching layer for embeddings.
 import json
 import sqlite3
 from pathlib import Path
-from typing import List, Optional, Tuple
 from datetime import datetime, timedelta
 
 
@@ -78,7 +77,7 @@ class EmbeddingCache:
     def set(
         self,
         hash_key: str,
-        embedding: List[float],
+        embedding: list[float],
         model: str
     ) -> None:
         """
@@ -103,7 +102,7 @@ class EmbeddingCache:
 
         self.conn.commit()
 
-    def get(self, hash_key: str) -> Optional[List[float]]:
+    def get(self, hash_key: str) -> list[float] | None:
         """
         Retrieve embedding from cache.
 
@@ -146,7 +145,7 @@ class EmbeddingCache:
 
         return json.loads(embedding_json)
 
-    def get_batch(self, hash_keys: List[str]) -> Tuple[List[Optional[List[float]]], List[bool]]:
+    def get_batch(self, hash_keys: list[str]) -> tuple[list[list[float] | None], list[bool]]:
         """
         Retrieve multiple embeddings from cache.
 
@@ -214,7 +213,7 @@ class EmbeddingCache:
 
         self.conn.commit()
 
-    def clear(self, model: Optional[str] = None) -> int:
+    def clear(self, model: str | None = None) -> int:
         """
         Clear cache entries.