Keep chat history fully ephemeral in demo mode

[Ho1yShif]

Problem

History was still being persisted to disk even with DEMO_MODE=1.

Demo mode was only ephemeral at the session-store layer — get_sqlite_session_store() swaps in an in-memory SQLite DB so data/user/chat_history.db is never written. But three other per-turn/tool disk-write paths ignored demo mode entirely, so conversation-derived content kept landing on disk:

Path	What it wrote	When
turn_runtime._mirror_event_to_workspace	Full turn transcript → data/user/workspace/<capability>/<turn_id>/events.jsonl	Every turn (always-on culprit)
memory.trace.append	L1 trace of user queries / stated preferences → data/memory/trace/<surface>/<date>.jsonl	On RAG search or write_memory tool
memory.store.write_preference	L3 data/memory/L3/preferences.md	On write_memory tool

Fix

• Add a shared is_demo_mode() guard in deeptutor/services/demo (reuses the process-wide limiter, so the answer is consistent everywhere and honours test overrides).
• Skip each of the three disk writes above when demo mode is on. write_preference returns ApplyReport(accepted=False, reason="demo mode: memory not persisted") so the caller sees an honest no-op.

This makes the demo's "chat history stays ephemeral / never written to disk" contract actually hold across the whole surface, not just the SQLite store.

Testing

• New tests/services/demo/test_demo_ephemeral_persistence.py — each culprit path is asserted to be a no-op when DEMO_MODE=1 and to still write when it's off (regression guard).
• Full demo, memory, session, and turn-event suites pass (315+ tests). Pre-commit (ruff, ruff format, bandit, mypy) all green.

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