Add a defensive encoding check to feishu-cli-import and feishu-cli-write
skills. The check detects both U+FFFD replacement characters and invalid
UTF-8 byte sequences in Markdown files before importing to Feishu,
preventing corrupted characters from flowing into documents.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

飞书 API 创建 QuoteContainer 时会自动在 index 0 生成空文本子块，
导致引用块顶部多出一个空行。此前仅对 Callout 做了清理，现扩展到
QuoteContainer 复用同一逻辑。

fix(import): 清理 QuoteContainer 自动生成的空子块

fix: add UTF-8 encoding validation before Feishu document import

…oken issues

- Wrap AddBoard and ImportDiagram calls in DoWithRetry for 429 retry
  with exponential backoff (fixes #88)
- Add --timeout flag to file download command, support custom timeout
  for DownloadFile/DownloadMedia/DownloadFromURL (fixes #79)
- Add --user-access-token support to sheet get/meta/read-plain/read-rich/
  find/write/append commands and underlying API functions (fixes #77)
- Extract firstString and resolveTimeout helpers to reduce boilerplate

…download timeout

- All 30 sheet commands now support --user-access-token for accessing
  spreadsheets without app-level permissions (25 client functions + 20 CMD files)
- Retry logic now propagates HTTP response headers from Feishu SDK, enabling
  precise rate-limit backoff using x-ogw-ratelimit-reset instead of blind jitter
  (7 client functions + 12 caller files updated)
- Unified --timeout flag for media download, doc media-download, and
  msg resource-download commands (default 5m, Go duration format)
- Fixed DownloadMessageResource using 30s timeout instead of 5m (bug)
- Updated 5 skill docs (toolkit, export, msg, import, board) to reflect changes