feat: add /export slash command for session Markdown

[Ayush7614]

Summary

• Adds /export [path] to render the current session to clean Markdown (closes Add /export — render session to Markdown #32).
• Tool calls use <details> summaries; edit diffs become fenced diff blocks; cost/request footer included.
• Default path: ./klaatai-session-<id>.md. /share is kept as an alias.
• Pure renderer in src/screens/export-session.ts with unit tests.

Test plan

• bun run typecheck
• bun test (186 pass on this branch)
• bun run bench:selfcheck
• bun run build
• In TUI: run a short turn with a tool call, /export, open the written .md and confirm tool/diff formatting

[Ayush7614]

cc: @aakashrajput

[github-actions]

🤖 KlaatAI Review Bot (powered by Klaatu, advisory only — a maintainer makes the real call)

Issue match
Fully addresses #32. Implements /export [path], renders tools and diffs legibly without raw JSON, includes the cost footer, updates slash autocomplete/help, and retains /share as an alias.

Test coverage
src/screens/export-session.test.ts thoroughly covers the pure renderer (path resolution, system filtering, tool collapsing, diff fencing, error headings). However, it lacks an integration test verifying that repl.ts correctly parses the [path] argument and writes the file via writeFileSync.

Correctness concerns
In src/screens/export-session.ts, renderToolBlock wraps tool output in triple backticks without escaping any backticks present in msg.content. If a tool outputs markdown code blocks, the generated export will have broken fencing. Consider dynamically adjusting the fence length (e.g., using ````) or escaping inner backticks.

Verdict
Needs a specific fix first: handle nested backticks in tool output to prevent broken markdown fencing in the exported file.

_{This is an automated review to help triage faster, not a gate. Nothing here blocks merging.}

[Ayush7614]

Addressed the review-bot note in a follow-up commit:

• Tool/diff bodies now use a fence longer than any backtick run inside the content (fenceFor), so nested markdown code blocks can’t break the export

[github-actions]

🤖 KlaatAI Review Bot (powered by Klaatu, advisory only — a maintainer makes the real call)

Issue match
Fully addresses #32. Implements /export [path] with the requested default path, collapsed tool <details>, fenced diff blocks, and cost footer. The --html bonus was correctly deferred. /share is preserved as an alias.

Test coverage
export-session.test.ts thoroughly covers the pure renderer: path resolution, nested backtick fence escaping, turn rendering, tool details, diff blocks, and error states. Missing coverage is the REPL integration (e.g., verifying /export triggers the file write and prints the path, or that the autocomplete menu includes the new command), though the core logic is well-tested.

Correctness concerns
In export-session.ts, resolveExportPath returns the trimmed argument directly without path.resolve(). While writeFileSync resolves relative paths against process.cwd(), this makes path resolution implicit and diverges from the comment's intent. Additionally, the default export location changed from homedir() (old /share) to cwd (new /export); this matches #32 but is a silent behavior change for existing /share users.

Verdict
Ready to merge as-is.

_{This is an automated review to help triage faster, not a gate. Nothing here blocks merging.}