feat: add read-only plan mode

[Radiummm]

Summary

• add a built-in /plan extension and toolbar toggle
• persist plan mode per session and restore the previous tool selection on exit
• expose only known read-only exploration tools while planning
• block direct shell commands and unknown extension tools from bypassing plan mode
• add English, Simplified Chinese, and Traditional Chinese labels

Tests

• node --experimental-strip-types --test lib/plan-mode-extension.test.mjs
• node_modules/.bin/tsc --noEmit
• npm run lint
• npm test (846 passed)

[agegr]

The existing read-only tool preset covers the main use case, so we are closing this PR to avoid adding a separate mode and state management. Any stricter read-only restrictions should build on the existing preset.