Adopt APM-distributed AI assets (Copilot code review already enabled)

Part of elastic/observability-robots#5115.

## Request

Copilot code review is already enabled here (`branch-protections/oblt-cli-buildkite-plugin/copilot.tf`). This repo just needs AI-agent guidance (`AGENTS.md`/skills/instructions).

### Track B — APM-distributed AI assets
- [ ] Add root `apm.yml` (`type: hybrid`, `compilation.target: all`) depending on `elastic/observability-robots/agents/common`, pulling in `shell.instructions` (this is a Buildkite plugin built on shell hooks) plus general code-review/testing skills.
- [ ] Add `.github/workflows/apm-update.yml` (pattern from `elastic/oblt-framework`, `microsoft/apm-action`, weekly schedule + `workflow_dispatch`).
- [ ] Run `apm compile` locally once, commit the generated `AGENTS.md`.

## Impact
Low-risk, small repo — good candidate to validate the APM rollout workflow before the larger repos.

## Stakeholders
Developer Experience / Engineering Productivity DX team.

Start by reviewing branch-protections/oblt-cli-buildkite-plugin/copilot.tf and the requested APM patterns from elastic/oblt-framework and microsoft/apm-action. Add the root apm.yml, .github/workflows/apm-update.yml, and generated AGENTS.md, then run apm compile locally. Done means the workflow and generated guidance are committed with the requested dependencies and weekly/manual update triggers.

Written by the indexing model from the issue text.