feat(ci): add prompt and managed agent cloud e2e pipeline

[Zhijie Huang (hund030)]

Summary

Add public-repo cloud E2E coverage for azd prompt and managed agents alongside hosted-agent validation. The pipeline runs changed samples on PRs and supports scheduled and manual full runs; Azure DevOps registration and resource authorization are separate setup steps.

Changes

• .azure-pipelines/prompt-agents-samples-ci.yml: discover, deploy, invoke, recover, and summarize samples under samples/cli/prompt-agents, with SKIP_PROVISION selecting existing or disposable resources.
• prompt-agents scripts and contracts: validate capabilities, track owned resources, and fail closed on missing evidence or unverified cleanup; fixture paths follow the CLI layout.
• Shared fixture tooling and docs: support CLI prompt-agent contracts without changing hosted-agent defaults, with credential-free regression coverage and activation guidance.

Manual Validation

• Ran 01-basic provisioning in westus2: azd init succeeded and ARM started provisioning, but the test identity lacked Microsoft.Authorization/roleAssignments/write; deleted the partial resource group and independently confirmed it was absent. Full deploy/invoke remains unverified.
• After the CLI sample move, ran the complete local prompt harness with Git Bash and jq (74 tests passed), plus contract discovery. Azure DevOps registration and PR triggering remain unverified.