docs: scope presence opt-out to internal helper agents

[ark-archastro]

Review on ArchCode

Problem and author intent

Internal review agents, summarizers, and judges can run the same ArchDev presence hooks as user-facing sessions. The distributed skill did not tell their launchers to use the opt-out introduced in ArchDev 0.46.9, so these helpers could appear as separate agents or overwrite inherited session attention.

What changed

• Teach the source skill and monitor reference to set ARCHDEV_PRESENCE_DISABLED=1 in the helper child environment before startup hooks run.
• Keep parent sessions and independently tracked workers reporting normally; explain native delegation tools that cannot set child environment variables.
• Document disabled command results, unchanged room logging, and normal expiry of existing rows.

Scope: Documentation only, in the public skill source. Risk: Low; this documents an existing CLI capability. User impact: Agents that load the updated skill receive explicit instructions for keeping internal helpers out of presence.

Testing

• Skill frontmatter validation and git diff --check passed.
• Independent review found no actionable issues; examples were checked against the 0.46.9 implementation.
• No executable behavior or tests changed. The Installer Smoke Test workflow runs on PRs but does not validate agent adherence to prose. A runtime end-to-end test cannot establish that an arbitrary harness follows these instructions; the guidance explicitly documents the native-delegation limitation.

Follow-ups

Scopey integration and other repository guidance are separate changes. Installed skill copies are not edited by this PR.