Default interactive chat to concise with deep-dive on request

[williamthorsen]

Part of #918 (epic).

Problem

Interactive chat is the one artifact with no save step, so the epic's compose-time templates don't reach it. It needs an equivalent concision default expressed as a behavioral rule.

Proposed solution

Add a behavioral concision rule to collaborate: default to reasoning silently and surfacing a concise answer or recommendation, with a deep dive only on request. Reference the concision spine; the ambient ## Concision line is the always-on backstop. Guard the rule so concision never overrides critical evaluation — a flaw, risk, or dissent worth raising is signal, not noise.

Acceptance criteria

• collaborate defaults to concise output with a deep-dive opt-in.
• The rule references the spine.
• The rule preserves critical evaluation: substantive concerns are never cut for brevity.