[product-manager-impersonator] Declarative workflow metadata manifest

## Feature Idea

**Summary:** Add a per-workflow `meta.yml` manifest that declares dogfood behavior (for example excluded-from-dogfood and auto-remediation chaining) so maintainers don’t need to edit hardcoded lists in `scripts/dogfood.sh`.

## Why a Customer Would Want This
Maintainers adding or changing workflows want one obvious place to declare workflow behavior. Today, behavior is split between workflow directories and script-level arrays, which makes onboarding and maintenance error-prone and increases review churn when a new workflow needs special handling.

## Rough Implementation Sketch
- Add optional `gh-agent-workflows//meta.yml` with a tiny schema (for example: `dogfood.exclude: true|false`, `dogfood.auto_remediation: true|false`).
- Update `scripts/dogfood.sh` to derive exclusion and remediation behavior from each workflow’s metadata, with sensible defaults when metadata is absent.
- Add a lightweight validator script (invoked from CI/lint) to fail on malformed metadata keys/values.
- Document the manifest and examples in `gh-agent-workflows/DEVELOPING.md`.

## Why It Won't Be That Hard
This is a small, contained change: one sync script, one validator, and docs. No compiler changes are required, and the behavior already exists today in hardcoded arrays, so this is mainly moving existing logic to a declarative source of truth.

## Evidence
- Hardcoded dogfood exclusions live in `scripts/dogfood.sh` (`EXCLUDED_WORKFLOWS` at lines 20-36, consumed at lines 60-67).
- Hardcoded auto-remediation set lives in `scripts/dogfood.sh` (`REMEDIATION_WORKFLOWS` at lines 40-49, consumed at lines 122-126 and appending chain at lines 151-160).
- Contributor docs explicitly depend on this implicit list behavior in `gh-agent-workflows/DEVELOPING.md` (for example lines 38, 118, 141, 201-203).
- Recent repo direction shows active investment in workflow reliability/maintainability (e.g., PRs #1706, #1710, #1680), making this a timely incremental improvement.

## Duplicate Check Notes
- Checked `/tmp/previous-findings.json`; related ideas exist (e.g., scaffolding/linting/setup improvements), but none propose a per-workflow declarative metadata manifest to replace `dogfood.sh` behavior lists.
- Targeted open-issue searches for `dogfood metadata` returned no open matches.

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Start by reading scripts/dogfood.sh, especially EXCLUDED_WORKFLOWS, REMEDIATION_WORKFLOWS, and their consumers at the cited lines, then review gh-agent-workflows/DEVELOPING.md. Define the manifest schema and defaults, add validation through the existing CI/lint path, and verify that metadata reproduces exclusion and auto-remediation behavior without hardcoded workflow lists.

Written by the indexing model from the issue text.