* docs(acp): add ACP agents onboarding/config guide
A guide for onboarding and configuring external ACP agents (Claude Code, Codex,
Gemini CLI): where each stores its login, which credentials onboarding collects,
and how subscription/login vs API-key precedence works.
The credentials-collection code originally in this PR landed via #1002
(registry-derived fields including Gemini), so #972 is now docs-only — rebased
onto current main.
* docs(acp): state that a subscription login takes priority over API keys
Make explicit that a subscription / OAuth login is preferred over an env API
key for all three providers — while signed in, a key isn't used (the onboarding
fields do nothing). Corrects the earlier claim that Claude Code uses
ANTHROPIC_API_KEY instead of the login: with both present, `claude auth status`
reports it's using the subscription (claude.ai). The base URL remains the one
override that takes effect under a login.
* docs(acp): show the conversation-turns edge in the flow diagram
Canvas relays each turn through the Agent Server, not just the settings PATCH;
add that edge so the diagram doesn't read as config-only (addresses review).
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Co-authored-by: Debug Agent <debug@example.com>
Hiep Le