feat(workflow-metrics): add reusable metrics workflow

[Hweinstock]

Problem

AgentCore workflows want to post metrics about the result of a workflow to setup alarms, and connect with internal ticketing.

Solution

• Add workflow-metrics.yml which can be run after specific workflows to post a duration metric.
• Document caller inputs and IAM permissions.

Verification

• actionlint with narrow ignores for the new GitHub job.workflow_repository and job.workflow_sha contexts not yet recognized by local actionlint 1.7.11, but is released (see https://github.blog/changelog/2026-09-03-github-actions-early-september-2026-updates/).
• tested through my personal fork on feat(ci): emit canary workflow duration metrics Hweinstock/agentcore-cli#59, by adding a temporary push trigger there, then triggering this workflow on my fork.

Actions run

Observed metric:

- Namespace: `AgentCoreCLI/Workflows`
- Metric: `WorkflowRunDuration`
- Value: `0 Seconds`
- Dimensions: `WorkflowName=workflow-metrics-smoke`, `Result=success`, `OS=Linux`
- Sample count: `1`