[nightly-maintenance] Clarify beta agent doc pointer

[r3dbars]

Summary

• Clarifies the Sources/Beta/CLAUDE.md pointer in docs/agent-onboarding.md so agents see the current empty BETA_BUILD shell first.
• Keeps the archived beta worker framed as handoff context, not active app behavior.

Verification

• bash scripts/dev/agent-preflight.sh
• git diff --check

[r3dbars]

verdict: no-findings-but-human

findings: none. This is a docs-only pointer update and matches Sources/Beta/CLAUDE.md current beta-config truth.

actions: no fix pushed.
current_head_sha: 24055e5
fix_commit: none
checks: repo-hygiene pass; gh pr view; gh pr diff --patch; gh pr checks; git diff --check origin/main...origin/codex/nightly-maintenance-beta-doc-current-20260530; AGENT/Beta docs reviewed.

manual_merge_handoff:
head_sha: 24055e5
verdict: no-findings-but-human
blockers: draft PR
checks: repo-hygiene pass; PR metadata/diff/checks reviewed; diff-check clean; Beta docs reviewed