docs: close quickstart onboarding safety gaps

[Nickfost]

Summary

Closes the remaining onboarding-safety gaps identified while reconciling merged quickstart PR #49:

• requires an amd64 or arm64 Docker host before installation;
• requires auditing and cancelling every queued job that could match the shared pool label before repository authorization, with an empty proof repository/group as the fail-closed fallback;
• scopes Docker residue checks to the repository, run ID, and run attempt while preserving slash-anchored container names;
• removes a duplicated phrase introduced during concurrent documentation reconciliation;
• adds a small runnable quickstart contract check and wires it into repository validation.

The change is documentation and validation-test only. It does not modify hosts, controllers, runners, capacity, runner groups, VMs, deployment infrastructure, or production.

Validation

• TDD RED: scripts/test_quickstart.py failed on the missing architecture prerequisite before the documentation fix;
• git diff --check: passed;
• shell syntax checks: passed;
• Python compilation: passed;
• planner, desired-state, health, quickstart, capacity-preflight, and installer tests: passed;
• fast/full plan expansion: passed;
• committed-secret scan: passed;
• full Docker build validation is delegated to pull-request CI.

Advances #24. Follow-up to #49.

[Nickfost]

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[Nickfost]

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Codex Review: Didn't find any major issues. Another round soon, please!

Reviewed commit: f8b329dfd7

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