[codex] Render Zhihu LaTeX equations

[chaochaoweb3]

Fixes #175.

Summary

• convert inline and block LaTeX delimiters into Zhihu equation image tags during Zhihu content transformation
• keep equation images inline instead of wrapping them in normal image figures
• skip LaTeX replacement inside pre/code blocks

Validation

• npm exec --yes --package=pnpm@10.17.0 -- pnpm --filter @wechatsync/core exec vitest run src/adapters/platforms/tests/zhihu.test.ts
• npm exec --yes --package=pnpm@10.17.0 -- pnpm --filter @wechatsync/core run typecheck
• git diff --check