fix: honor CLAUDE_CONFIG_DIR environment variable (#261)

Fixes #255

- Add getClaudeConfigDir() utility function that respects CLAUDE_CONFIG_DIR env var
- Update all hardcoded ~/.claude paths to use the new utility
- Add comprehensive tests for getClaudeConfigDir()
- Maintain backward compatibility with default ~/.claude when env var is not set

Files updated:
- src/shared/claude-config-dir.ts (new utility)
- src/shared/claude-config-dir.test.ts (tests)
- src/hooks/claude-code-hooks/config.ts
- src/hooks/claude-code-hooks/todo.ts
- src/hooks/claude-code-hooks/transcript.ts
- src/features/claude-code-command-loader/loader.ts
- src/features/claude-code-agent-loader/loader.ts
- src/features/claude-code-skill-loader/loader.ts
- src/features/claude-code-mcp-loader/loader.ts
- src/tools/session-manager/constants.ts
- src/tools/slashcommand/tools.ts

Co-authored-by: sisyphus-dev-ai <sisyphus-dev-ai@users.noreply.github.com>

src/shared/claude-config-dir.ts

@@ -0,0 +1,11 @@
+import { homedir } from "node:os"
+import { join } from "node:path"
+
+export function getClaudeConfigDir(): string {
+  const envConfigDir = process.env.CLAUDE_CONFIG_DIR
+  if (envConfigDir) {
+    return envConfigDir
+  }
+  
+  return join(homedir(), ".claude")
+}