Fix studio status to show full markdown-formatted summary

Changes:
- Fix JSON path for session logs: now reads from context_window.conversation_history
  (with fallback to messages for backwards compatibility)
- Remove 500-character truncation to show full summary
- Add termimad dependency for terminal markdown rendering
- Display summary with proper markdown formatting (headers, bold, code, lists)

The extract_session_summary() function was looking for messages at the wrong
JSON path. Session logs store conversation history at context_window.conversation_history,
not at the top-level messages key.

crates/studio/Cargo.toml

@@ -16,3 +16,4 @@ serde = { workspace = true }
 serde_json = { workspace = true }
 uuid = { workspace = true }
 chrono = { version = "0.4", features = ["serde"] }
+termimad = "0.31"