fix(tui): soften syntax colors for calls and builtins

[opencode-agent]

Summary

• map function calls and function builtins to syntaxFunction instead of syntaxVariable/error
• render variable parameters and common variable/module builtins as normal text
• cover the TUI syntax mapping with a focused theme-store test

Tests

• bun run test test/cli/tui/theme-store.test.ts
• PATH="/tmp/opencode/bin:/home/daytona/.opencord/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/local/bun-node-fallback-bin" git push -u origin fix/tui-syntax-builtins (pre-push ran bun turbo typecheck)