fix: write valid multi-line tmux config in Pi Agent images

The Pi Agent dockerfile templates created ~/.tmux.conf with a literal
\n because the RUN command used single-quoted echo. Tmux never parsed
the malformed line, so mouse mode stayed off. Without tmux mouse mode,
mouse-wheel events in xterm.js fell back to Up/Down arrow keys and
cycled shell command history instead of scrolling the terminal buffer.

- Use printf '%s\n' to write real newlines in .tmux.conf.
- Apply the same fix to the ranger rc.conf where the same bug existed.
- Update tool-images/pi-agent.dockerfile and both affected alembic
  migration dockerfile strings.

Quality gates: npm run typecheck, npm run lint, npm test -- --run (87 passed),
py_compile on changed migrations.

Refs: openspec/changes/fix-tmux-mouse-config

apps/api/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: apps/api
 
 ## role
-Self-hosted FastAPI backend API that manages projects, git repositories, and development tools via Docker instances.
+FastAPI backend API that manages projects, git repositories, and development tools via Docker instances.
 ## parent
 index: apps/.pi-map.index.md
 map: apps/.pi-map.md