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/features/claude-code-agent-loader/loader.test.ts

@@ -191,31 +191,18 @@ describe("claude-code-agent-loader", () => {
   describe("loadUserAgents", () => {
     test("returns empty object when pointed at dir without agents/", () => {
       const root = trackDir(mkdtempSync(join(tmpdir(), "agent-loader-test-")))
-      // Temporarily set env var — best-effort in parallel test runner
-      const prev = process.env.CLAUDE_CONFIG_DIR
-      try {
-        process.env.CLAUDE_CONFIG_DIR = root
-        const result = loadUserAgents()
-        expect(result).toEqual({})
-      } finally {
-        if (prev !== undefined) process.env.CLAUDE_CONFIG_DIR = prev
-        else delete process.env.CLAUDE_CONFIG_DIR
-      }
+      process.env.CLAUDE_CONFIG_DIR = root
+      const result = loadUserAgents()
+      expect(result).toEqual({})
     })
   })
 
   describe("loadOpencodeGlobalAgents", () => {
     test("returns empty object when pointed at dir without agents/", () => {
       const root = trackDir(mkdtempSync(join(tmpdir(), "agent-loader-test-")))
-      const prev = process.env.OPENCODE_CONFIG_DIR
-      try {
-        process.env.OPENCODE_CONFIG_DIR = root
-        const result = loadOpencodeGlobalAgents()
-        expect(result).toEqual({})
-      } finally {
-        if (prev !== undefined) process.env.OPENCODE_CONFIG_DIR = prev
-        else delete process.env.OPENCODE_CONFIG_DIR
-      }
+      process.env.OPENCODE_CONFIG_DIR = root
+      const result = loadOpencodeGlobalAgents()
+      expect(result).toEqual({})
     })
   })