[product-manager-impersonator] Project Summary: optional Agentic Activity Digest

## Feature Idea

**Summary:** Add an optional `include-agentic-activity` input to `gh-aw-project-summary` that appends a compact “Agentic Activity Digest” section (bot-created issues/PRs and no-op volume) without changing existing human-focused sections.

## Why a Customer Would Want This
Maintainers using many scheduled workflows need quick confidence in what automation actually did. Today, project summaries intentionally exclude bot activity, while no-op signals are tracked elsewhere, so maintainers must stitch context manually.

## Rough Implementation Sketch
- Add a new boolean input (default `false`) in `.github/workflows/gh-aw-project-summary.md`.
- When enabled, gather a small set of bot-activity metrics (recent bot-created issues/PRs, no-op count) using existing GitHub search patterns already used by the workflow.
- Append a concise `### 🤖 Agentic Activity Digest` section only when data exists.
- Document the input in `gh-agent-workflows/project-summary/README.md` (or the generated docs surface used for this workflow).

## Why It Won't Be That Hard
This is an additive change to one workflow prompt/spec plus docs: the workflow already performs GitHub searches and already writes a structured issue. The new section can reuse that existing machinery with a small, gated branch.

## Evidence
- `.github/workflows/gh-aw-project-summary.md:95-107` already collects issues/PRs via `github-search_*` and can reuse that mechanism for digest metrics.
- `.github/workflows/gh-aw-project-summary.md:151` explicitly excludes bot-authored activity from counts/sections, creating a visibility gap for automation outcomes.
- `.github/workflows/gh-agent-workflows.md:66-101` shows many scheduled workflows running regularly, increasing need for a compact automation digest.
- `docs/blog/posts/silence-is-better-than-noise.md:33-37` emphasizes signal quality and that no-op is success, which this digest would preserve while improving visibility.
- Existing tracking issue `#448` (`[aw] No-Op Runs`) shows no-op data exists but is separated from daily project-summary reporting.

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[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Product Manager Impersonator](https://github.com/elastic/ai-github-actions/actions/runs/22996885348)

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