fix(ci): allow cache save within job timeout

[i9wa4]

Summary

Retire the redundant ci workflow and retain ci-nix-quick-install as the single CI workflow. This resolves the recurring cache-save cancellation without deleting validation coverage.

Authorization

The user explicitly authorized creating this issue in Messenger message 20260731-235041-sf40c-r683b-from-messenger-to-orchestrator.md.

Evidence

• ci run 30633600591 on merged PR feat(subagent-review): converge iterative reviews #302 completed all substantive validation but was cancelled during Post Cache Nix store when the five-minute whole-job limit expired. The same pattern appears in run 30242951667.
• quick-install run 30633600546 on the same commit completed successfully in about two minutes, including cache save.
• The workflow source diff shows identical triggers, permissions, concurrency, checkout, cache action, nix flake check, Agent Skills dry-run, and gitleaks/betterleaks scans. The only functional difference is the Nix installer and its distinct cache-key prefix.
• Commit 6bb8aeef introduced quick-install as a side-by-side installer speed/reliability experiment with otherwise identical coverage.

Scope

• Remove .github/workflows/ci.yaml.
• Retain .github/workflows/ci-nix-quick-install.yaml unchanged as the CI coverage owner.

Non-goals

• Removing validation, leak scans, caching, or concurrency cancellation
• Weakening CI guarantees
• Editing the retained quick-install workflow

Verification

• Run focused YAML/workflow and diff checks.
• Run repository checks, including nix run .#check.\n- Verify the pull-request workflow run completes with the retained quick-install CI.\n