Gap-4: Functions Operate (B → A) — Hosting plan guide, cold start playbook, slots

Parent: #1608

## Functions Operate: B → A

**Learning from:** App Service (B+) — deployment slots docs cover the slot lifecycle well

**Current state:** `azure-upgrade` skill covers Consumption→Flex. Missing: slot management for Premium/Dedicated, scaling best practices, cold start mitigation.

## Deliverables

- [ ] `services/functions/hosting-plans.md` — Comparison matrix: Consumption vs Flex vs Premium vs Dedicated vs Container Apps
- [ ] `services/functions/cold-start.md` — Mitigation strategies per plan (pre-warmed, always-ready, Flex minimum instances)
- [ ] `services/functions/slots.md` — Deployment slots for Premium/Dedicated, slot-sticky settings, swap best practices
- [ ] Extend `azure-upgrade` to cover Premium→Flex, Dedicated→Flex paths
- [ ] Add 1 integration test for Functions slot/swap scenario

> ⚠️ **Starting assessment** — domain experts should modify or contribute what they believe is essential.

Start by reading the existing azure-upgrade skill and the App Service deployment slots documentation referenced in the issue. Update services/functions/hosting-plans.md, cold-start.md, and slots.md, extend the listed upgrade paths, and add the Functions slot/swap integration test. Done means all listed deliverables are covered and the integration scenario passes.

Written by the indexing model from the issue text.