Add cloud agent instructions for issue-assigned agents (#2248)

### ❔What, Why & How

Adds `.github/agents/issue.instructions.md` — instructions for cloud
agents that get assigned to issues and create PRs.

**Basic PR requirements:**
- When to update `RELEASENOTES.md` (action/template/workflow changes,
bug fixes)
- Test expectations: Pester naming conventions, cross-platform
(PS5/PS7), mocking
- Documentation: settings in `Scenarios/settings.md` +
`settings.schema.json`, scenario docs
- Code quality: standard header, secret masking, encoding, JSON handling

**Repository architecture reference:**
- Action anatomy: `action.yaml` → `Invoke-AlGoAction.ps1` →
`<Action>.ps1` → `AL-Go-Helper.ps1`
- Template layout: PTE vs AppSource variants, which workflows are shared
- Module structure under `Actions/.Modules/`
- Test organization and CI matrix

**Step-by-step guides for adding features:**
- Adding/modifying settings (code → schema → docs → tests)
- Adding a new action (folder, action.yaml, script, tests, workflow
integration)
- Adding/modifying workflows (dual-template consistency, YAML
conventions)
- Modifying shared helpers (caller analysis, backward compat)

**Common pitfalls section** covering dual-template sync, path
separators, PS5/PS7 differences, schema drift.

**Clarifications added from review feedback:**
- Explicitly notes that `$errorActionPreference` (camelCase) is this
repo's convention, so agents don't confuse it with canonical PowerShell
casing.
- Clarifies the shell convention distinction: template workflows use
`powershell`, the repo's own CI workflows use `pwsh`.

### ✅ Checklist

- [x] Add tests (E2E, unit tests) — N/A, documentation-only change
- [x] Update RELEASENOTES.md — N/A, internal tooling, not user-facing
- [x] Update documentation (e.g. for new settings or scenarios) — this
*is* the documentation
- [x] Add telemetry — N/A

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: mazhelez <43066499+mazhelez@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>