Add /export — render session to Markdown

[aakashrajput]

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Why

Users want to share or archive what the agent did. Sessions are stored as JSONL — unreadable for humans.

What

New slash command /export [path]:

• Renders the current session to clean Markdown: user/assistant turns, collapsed tool calls (name + one-line summary, expandable details block), diffs as fenced ```diff blocks, timestamps, final token/cost stats footer
• Default path ./klaatai-session-<id>.md; path argument overrides
• Bonus (separate PR welcome): --html variant with inline styles

Where to start

• Session message history is already in memory in src/screens/repl.ts; session JSONL format in the session-save/resume code
• Markdown rendering knowledge lives in src/engine/widgets/markdown.ts (you're doing the reverse — model messages → md text)

Acceptance criteria

• /export writes the file and prints the path in the transcript
• Tool calls and diffs render legibly; no raw JSON dumps
• In slash autocomplete; typecheck + tests green

[Ayush7614]

Hi @aakashrajput — I created a PR for this: #45

Adds /export [path] to render the current session to clean Markdown (with /share kept as an alias).