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/features/claude-code-mcp-loader/loader.ts

@@ -1,6 +1,6 @@
 import { existsSync } from "fs"
-import { homedir } from "os"
 import { join } from "path"
+import { getClaudeConfigDir } from "../../shared"
 import type {
   ClaudeCodeMcpConfig,
   LoadedMcpServer,
@@ -16,11 +16,11 @@ interface McpConfigPath {
 }
 
 function getMcpConfigPaths(): McpConfigPath[] {
-  const home = homedir()
+  const claudeConfigDir = getClaudeConfigDir()
   const cwd = process.cwd()
 
   return [
-    { path: join(home, ".claude", ".mcp.json"), scope: "user" },
+    { path: join(claudeConfigDir, ".mcp.json"), scope: "user" },
     { path: join(cwd, ".mcp.json"), scope: "project" },
     { path: join(cwd, ".claude", ".mcp.json"), scope: "local" },
   ]