Adopt GitHub Agentic Workflows for issue, PR, and documentation automation

## Summary

Introduce [GitHub Agentic Workflows (`gh-aw`)](https://github.github.com/gh-aw/) to `Azure/git-ape` to automate repetitive maintainer tasks, keep documentation evergreen, and improve contributor experience — aligning with the repo's mission as a *platform engineering framework for the agentic age*.

Agentic workflows are authored in Markdown, compiled to hardened GitHub Actions via the `gh aw` CLI, and run with guardrails (sandboxed execution, scoped tokens, audit trails, output validation). They support multiple engines (Copilot, Claude, Codex).

## Motivation

- Reduce maintainer toil on triage, review, and docs upkeep.
- Dogfood agentic patterns in a repo explicitly about the agentic age.
- Provide a reference implementation contributors can learn from.

## Proposed workflows

### 🗂️ Issue management
- [ ] **Issue Triage Agent** — On `issues.opened` / `reopened`: classify, apply labels (`bug`, `feature`, `docs`, `question`), detect likely duplicates, request missing repro info, and post a welcome/next-steps comment.
- [ ] **Stale Issue/PR Groomer** — Weekly schedule: summarize aging items, ping assignees, and suggest closure for abandoned items (human-in-the-loop).

### 🔀 Pull Request management
- [ ] **PR Review Agent (Shell-focused)** — On `pull_request.opened` / `synchronize`: AI-assisted review tuned for Shell best practices (shellcheck-style issues, quoting, `set -euo pipefail`, error handling, portability).
- [ ] **PR Describer / Summarizer** — Auto-generate or enrich thin PR descriptions, suggest labels, and draft changelog entries from the diff.

### 📚 Documentation evergreen
- [ ] **Docs Drift Detector** — On push to `main`: detect when script behavior, CLI flags, or configuration diverge from `README.md` / docs and open a PR with proposed updates.
- [ ] **README & Examples Refresh** — Monthly schedule: validate that commands/snippets in README still run and refresh outdated examples.

### 🛠️ Repo maintenance
- [ ] **CI Failure Triager** — On workflow failure on `main`: analyze logs, identify root cause, open or update a tracking issue with a suggested fix.
- [ ] **Weekly Repo Health Report** — Scheduled digest issue summarizing new/closed issues, merged PRs, contributor activity, and flagged hotspots.

## Implementation plan

1. **Bootstrap**
- Install `gh aw` CLI locally for authoring/compiling.
- Add `.github/workflows/agentic/` directory for Markdown workflow sources.
- Document local authoring flow in `CONTRIBUTING.md`.
2. **Guardrails & secrets**
- Decide on engine(s): Copilot by default; allow opt-in to Claude/Codex.
- Configure least-privilege `GITHUB_TOKEN` scopes per workflow.
- Enable required-reviewer gates on any agent-opened PRs.
3. **Rollout (phased)**
- Phase 1: Issue Triage + PR Describer (low-risk, read/label focused).
- Phase 2: PR Review Agent + Docs Drift Detector.
- Phase 3: CI Failure Triager, Stale Groomer, Health Report, README Refresh.
4. **Observability**
- Enable workflow run summaries and agent audit logs.
- Track metrics: time-to-triage, PR review latency, doc drift PRs merged.

## Acceptance criteria

- [ ] `gh aw` bootstrapped and documented in `CONTRIBUTING.md`.
- [ ] At least Phase 1 workflows running on `main`.
- [ ] Each workflow has a short README describing intent, triggers, and how to disable.
- [ ] Maintainer opt-out path documented (labels like `no-agent` to skip automation).

## References

- [GitHub Agentic Workflows — Home](https://github.github.com/gh-aw/)
- [GitHub Blog: Automate repository tasks with GitHub Agentic Workflows](https://github.blog/ai-and-ml/automate-repository-tasks-with-github-agentic-workflows/)
- [Getting Started guide (josh-ops)](https://josh-ops.com/posts/github-agentic-workflows/)

## Labels (suggested)

`enhancement`, `automation`, `documentation`, `good first issue` (for individual sub-tasks if broken out)