fix(session): enable plugins in headless SDK env

[Mingholy]

Summary

• Enable Claude Code marketplace plugin sync for headless SDK session spawns by setting CLAUDE_CODE_SYNC_PLUGIN_INSTALL=1.
• Keep existing loop plugin preparation intact.
• Add regression coverage for the headless SDK spawn env.

Test plan

• bun test server/test/session-headless-env.test.ts server/test/plugin-installer.test.ts
• cd server && bunx tsc --noEmit -p tsconfig.json
• bun run build
• Lint: repository has no lint script or ESLint/Biome/Oxlint config.