test: remove redundant local env restoration

The global test-setup.ts (preloaded via bunfig.toml) already snapshots
process.env in beforeEach and restores it in afterEach for every test.
Per-file env tracking is duplicate work and creates four different
patterns for the same problem.

Affected files:
- src/shared/claude-config-dir.test.ts (beforeEach/afterEach pair)
- src/shared/plugin-command-discovery.test.ts (ENV_KEYS + envSnapshot)
- src/features/claude-code-agent-loader/loader.test.ts (try/finally)
- src/features/skill-mcp-manager/connection-env-vars.test.ts (ORIGINAL_ENV)

src/shared/claude-config-dir.test.ts

@@ -1,23 +1,9 @@
-import { describe, test, expect, beforeEach, afterEach } from "bun:test"
+import { describe, test, expect } from "bun:test"
 import { homedir } from "node:os"
 import { join } from "node:path"
 import { getClaudeConfigDir } from "./claude-config-dir"
 
 describe("getClaudeConfigDir", () => {
-  let originalEnv: string | undefined
-
-  beforeEach(() => {
-    originalEnv = process.env.CLAUDE_CONFIG_DIR
-  })
-
-  afterEach(() => {
-    if (originalEnv !== undefined) {
-      process.env.CLAUDE_CONFIG_DIR = originalEnv
-    } else {
-      delete process.env.CLAUDE_CONFIG_DIR
-    }
-  })
-
   test("returns CLAUDE_CONFIG_DIR when env var is set", () => {
     process.env.CLAUDE_CONFIG_DIR = "/custom/claude/path"