Prevent dashboard chat from loading private MEMORY.md

[ianalloway]

Motivation

• The dashboard channel was classified as a private/main session and therefore received MEMORY.md in the LLM system prompt, which exposed operator long-term memory to unauthenticated /api/chat requests.

Description

• Remove "dashboard" from the private/main channel tuple in build_chat_system_prompt so dashboard chat uses the shared prompt path and does not include MEMORY.md.
• Update the function docstring to clarify that only authenticated private channels receive memory.
• Add a regression test test_dashboard_chat_uses_shared_prompt_without_memory in tests/test_workspace.py that verifies CLI still gets memory but dashboard does not.

Testing

• Ran python -m pytest tests/test_workspace.py tests/test_dashboard_chat.py tests/test_server.py and all relevant tests passed (7 passed, 3 skipped).

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