Add `ember history export` command

[niklasmarderx]

What

Add a command to export chat history as JSON or Markdown.

Why

Users want to save interesting conversations, share them, or analyze their AI usage patterns.

How

1. ember-storage already stores conversations in SQLite
2. Add a new HistoryAction::Export variant in crates/ember-cli/src/main.rs
3. Query conversations from storage
4. Format as JSON (--json) or Markdown (--markdown, default)
5. Output to stdout or file (--output path)

ember history export                    # last conversation as markdown
ember history export --json             # as JSON
ember history export --all --output dir # all conversations to directory

Reference

crates/ember-storage/src/lib.rs has the conversation storage API.

[Wpelletier01]

I’m wondering if it might be preferable to use a single argument like --format <json|md|...> rather than separate flags such as --json and --markdown. This could make it easier to extend the CLI in the future with additional output formats. What do you think ?