feat: full-fidelity conversation export (chat + tool calls)

Chat only ever lived in SQLite's `exchanges` table (what was *said*); tool
calls were transient — logged to the in-memory ring buffer and gone at
end-of-turn. This adds a persistent record of what Lyra *did* and exports the
two merged into one transcript.

- memory: new `tool_events` table + `add_tool_event`/`tool_events` accessors;
  `delete_session` cascades to it.
- chat: persist each tool call (name/args/result) right where it fires, in both
  the non-stream and stream paths. Move the user `remember` to just after
  assembly so its timestamp precedes mid-turn tool events — keeps the export in
  true chronological order (and records when the message actually arrived).
- transcript: new module renders a session as Markdown (Brian/Lyra speech with
  ⚙ tool-call lines interleaved) or JSON (machine-readable event stream).
- server: GET /sessions/{id}/export?format=md|json (attachment download).
- ui: ⬇ Export button by the session selector (Markdown or JSON).

Doubles as the receipt for "did the tool actually fire?" — the thing that was
invisible when she'd reply about a hand without logging it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyra/web/static/index.html

@@ -88,6 +88,7 @@
       <select id="sessions"></select>
       <button id="newSessionBtn">➕ New</button>
       <button id="renameSessionBtn">✏️ Rename</button>
+      <button id="exportSessionBtn" title="Download full transcript (chat + tool calls)">⬇ Export</button>
       <button id="thinkingStreamBtn" title="Show live activity log">📜 Live Log</button>
     </div>
 
@@ -987,6 +988,19 @@
 	  addMessage("system", `Session renamed to: ${newName}`);
 	});
 
+	document.getElementById("exportSessionBtn").addEventListener("click", () => {
+	  if (!currentSession) { addMessage("system", "No session to export."); return; }
+	  const fmt = window.confirm("Export as Markdown? (Cancel = JSON)") ? "md" : "json";
+	  // Hitting the download endpoint navigates a hidden anchor so the browser
+	  // saves the file (chat + interleaved tool calls) instead of rendering it.
+	  const a = document.createElement("a");
+	  a.href = `${RELAY_BASE}/sessions/${encodeURIComponent(currentSession)}/export?format=${fmt}`;
+	  a.download = "";
+	  document.body.appendChild(a);
+	  a.click();
+	  a.remove();
+	});
+
       // Settings Modal
       const settingsModal = document.getElementById("settingsModal");
       const settingsBtn = document.getElementById("settingsBtn");