[Feature Request] Add im +messages-export for copy-friendly chat message export

Problem

Current lark-cli im +messages-mget can batch fetch messages as structured JSON, but there is no copy-friendly export command for selected chat messages.

In real Feishu/Lark workflows, users often need to select several chat messages and share them with designers, producers, PMs, or external collaborators as readable text or Markdown. The desktop client can display these messages, but reliably copying multiple messages is difficult, especially when messages include rich post content, replies/quotes, images, files, sender names, timestamps, and original message links.

Current workaround

Today the practical workflow is:

1. Use im +chat-messages-list or im +messages-search to locate message IDs.
2. Use im +messages-mget --message-ids "om_xxx,om_yyy" to batch fetch full message JSON.
3. Write ad-hoc parsing logic to convert the JSON into human-readable Markdown/text.

This works for agents and scripts, but every workflow has to re-implement the export formatting.

Proposal

Add a shortcut:

lark-cli im +messages-export

The goal is to export selected messages into copy-paste friendly Markdown/text while preserving enough structure for traceability.

Suggested input modes

Fetch by explicit message IDs:

lark-cli im +messages-export --message-ids "om_xxx,om_yyy,om_zzz"

Fetch by chat and time window:

lark-cli im +messages-export \
  --chat-id oc_xxx \
  --start 2026-04-08T00:00:00+08:00 \
  --end 2026-04-10T23:59:59+08:00

Optionally, search then export:

lark-cli im +messages-export --chat-id oc_xxx --query "keyword"

Suggested output formats

--format markdown
--format text
--format json

markdown and text should be optimized for direct copy/paste into chat, docs, and reports. json should preserve the structured result for scripts.

Expected exported fields

For each message, include when available:

• sender display name
• send time
• message text / post content
• reply or quote context
• image, file, link placeholders or downloaded paths when requested
• message_app_link
• chat_id or chat name if available
• message_id for traceability

Example Markdown output shape:

### 2026-04-09 21:20 | Alice

Replying to Bob:
Please use this version.

The fabric on the body should be changed to silk, or removed directly because it looks uncomfortable.
Add some shiny colored scales to the tail so the tail color has more variation.

Attachments:
- image: img_xxx

Source:
- message_id: om_xxx
- link: https://...

Non-goals

• This does not need to simulate Feishu/Lark desktop client multi-select UI.
• OCR for images is not required.
• Merge-forward is useful, but it is not the same as exporting messages as copyable text.

Why existing commands are not enough

+messages-mget is useful for structured JSON, but users still need a readable "copy this to someone" format. A dedicated export shortcut would reduce repeated ad-hoc parsing in agents and scripts.

Environment where this was observed

• lark-cli version 1.0.40
• im +messages-mget exists and supports batch fetching by comma-separated message IDs.
• im messages forward, im messages merge_forward, and im threads forward exist, but they are forwarding workflows, not copy-friendly text export workflows.

Start by reading the existing im +messages-mget, im messages forward, im messages merge_forward, and im threads forward command entry points to understand message retrieval and output conventions. Define the supported input and format scope before implementation; done means selected messages can be exported as readable Markdown or text with the requested traceability fields, while JSON remains structured.