Many users paste Reasonix transcripts into GitHub issues, internal
wikis, or design docs when filing a bug or sharing an investigation.
There was no way to get a transcript out of the desktop app — the
data lives in the Webview's React state and the on-disk session
files are a bespoke format.

Add lib/exportSession.ts: a pure function that renders the items
array to a Markdown document. The shape matches what a reader
expects:

  # <title>
  - model: <ref>
  - workspace: <cwd>
  - started: <iso>
  - ended: <iso>
  - duration: 12m 34s

  ## User
  ...

  ## Assistant
  ...

  ### `tool_name`
  ```json
  { ...args }
  ```
  **output**
  ```
  ...tool output...
  ```
  **error** (if any)
  ```
  ...error body...
  ```

Sub-agent calls (parentId-bearing tool items) are rendered nested
under the parent task call. Todo panels and exit_plan_mode cards
are skipped — they were already shown live in the UI and the
plan-mode approval card itself, so re-emitting them would
duplicate. Notices are ephemeral. Compaction boundaries are
emitted as a blockquote with a collapsible summary <details>.

Two ways to use it from the topbar:

  - Single click:  copies the rendered Markdown to the clipboard
                    (the common case — paste into Slack/GitHub).
  - Double click: triggers a Blob download named
                    reasonix-YYYY-MM-DD-HHMM.md. Works in the
                    Wails WebView (the OS save dialog surfaces
                    for the download).

A future SaveTextFile bridge binding can route to a native
SaveFileDialog without changing the rendering path.