fix(newos4): connect to Ollama on first use, honest write_file, list_files sees /sys

Three defects, each re-probed first and covered by a test that is red on HEAD.

- ollama-manager.js: nothing connects at boot; only the Ollama tab button called connect(),
  so a user's first message died with "Ollama not connected. Use ollamaconnect first." even
  when the endpoint was healthy. New _ensureConnected() runs from chat(), chatStream() and
  chatWithTools(): one attempt, shared by concurrent callers, a "Connecting…" hint, and only
  then the error — which now says the endpoint is unreachable instead of telling the user to
  run a command they already expected to work.
- module-aiassist.js write_file tool: fs.writeFile()'s false return was dropped and the tool
  answered "File written: …" anyway, sending the model on to a read that found nothing
  (MAINT-012). It now returns "Write FAILED: … — parent directory missing or target is a
  directory; nothing was written".
- module-aiassist.js list_files tool: the hand-rolled walk over root.content never resolved
  the virtual mounts, so /sys — a directory the UI and `ls` both show — answered
  "Directory not found" (MAINT-011). It now uses fs.readDir(), which resolves mounts and
  returns null for a genuinely missing path.

Tests: modules/test/test-ai-fs-tools.js (7 asserts) and
modules/test/test-ollama-autoconnect.js (11 asserts). RED on HEAD: ai tools 4 passed/3 FAILED
(/sys → "Directory not found", write_file → "File written: …"), ollama 2 passed/9 FAILED
("Use ollamaconnect first"). GREEN now 7/7 and 11/11; maintain --fix reports "node tests 9/9 pass".
Live on v4.6.105: connectedBefore=false, _ensureConnected()=true in 41 ms, 37 models.

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