feat: use Azure Deployment Stacks for idempotent destroy (#30)

[arnaudlh]

Replace plain subscription deployments (az deployment sub create) and RG-based destroy (az group delete) with Azure Deployment Stacks across all workflows and agent docs.

Why:
Plain sub-deployments + RG-delete leave orphans when a template spans
multiple RGs or creates sub-scope resources (role/policy assignments,
cross-scope role grants, remote-side refs). Stacks track every managed
resource across every scope as a single lifecycle unit, so destroy is
one idempotent call regardless of how templates evolve. See Azure#30.

Workflow changes:

• git-ape-deploy: az stack sub validate/create with --action-on-unmanage deleteAll --deny-settings-mode none. Captures stackId and managedResources[] into state.json (schema v1.0).
• git-ape-destroy: az stack sub show (inventory) + az stack sub delete --action-on-unmanage deleteAll --bypass-stack-out-of-sync-error. Removed old Build-destroy-plan / sub-resource-sweep / RG-delete steps. Missing-stack path now records already-destroyed (idempotent).
• git-ape-plan: az stack sub validate for validation; keep az deployment sub what-if (stacks do not support what-if yet).

Docs:

• copilot-instructions.md: updated deploy/destroy descriptions and inline bash example to stacks commands.
• azure-resource-deployer.agent.md: deploy and rollback patterns use az stack sub create/delete.
• azure-template-generator.agent.md: CLI and PowerShell examples use deployment stacks.
• git-ape.agent.md: Stage 3 description references stacks.
• DEPLOYMENT_STATE.md: documents state.json schema v1.0 with stackId and managedResources[] fields.