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-agent-loader/loader.ts

@@ -1,9 +1,9 @@
 import { existsSync, readdirSync, readFileSync } from "fs"
-import { homedir } from "os"
 import { join, basename } from "path"
 import type { AgentConfig } from "@opencode-ai/sdk"
 import { parseFrontmatter } from "../../shared/frontmatter"
 import { isMarkdownFile } from "../../shared/file-utils"
+import { getClaudeConfigDir } from "../../shared"
 import type { AgentScope, AgentFrontmatter, LoadedAgent } from "./types"
 
 function parseToolsConfig(toolsStr?: string): Record<string, boolean> | undefined {
@@ -68,7 +68,7 @@ function loadAgentsFromDir(agentsDir: string, scope: AgentScope): LoadedAgent[]
 }
 
 export function loadUserAgents(): Record<string, AgentConfig> {
-  const userAgentsDir = join(homedir(), ".claude", "agents")
+  const userAgentsDir = join(getClaudeConfigDir(), "agents")
   const agents = loadAgentsFromDir(userAgentsDir, "user")
 
   const result: Record<string, AgentConfig> = {}