docs(plan): add phase 1.5 CI stabilization checklist and handoff

[rendall]

Summary

This PR adds and finalizes Phase 1.5 planning/checklist artifacts to stabilize CI test gating between Phase 1 and Phase 2.

Why

After the Phase 1 merge, CI stability required an explicit bridge phase:

• keep backend CI green and predictable,
• document temporary frontend test-gating defer,
• hand off deterministic frontend locale test restoration to Phase 3.

Changes

• Added Phase 1.5 plan:
  • docs/plans/phase-01-5-ci-stabilization-and-test-gating.md
• Added Phase 1.5 checklist:
  • docs/plans/phase-01-5-checklist.md
• Completed 3-pass checklist review updates (QC/integration/sanity) in the Phase 1.5 plan.
• Added explicit Phase 1.5 handoff note to Phase 3 plan:
  • docs/plans/phase-03-test-determinism-and-ci-hardening.md
• Updated plans index sequencing and links:
  • docs/plans/README.md
• Archived completed Phase 1 docs:
  • docs/archive/phase-01-checklist.md
  • docs/archive/phase-01-dependency-and-platform-upgrade.md

Scope

Docs/planning only. No runtime/code behavior changes.

Validation

• Reviewed checklist against docs/norms/checklist.md conventions:
  • atomic/checkable items,
  • explicit dependencies,
  • behavior-slice mapping.
• No test execution required for this docs-only PR.