[product-manager-impersonator] Single-workflow compile target

## Feature Idea

**Summary:** Add an optional targeted compile command (`make compile-workflow WORKFLOW=`) so maintainers can regenerate only one workflow’s generated files during edits.

## Why a Customer Would Want This

Workflow maintainers usually touch one workflow at a time, but today the documented path always recompiles and re-syncs the entire catalog. That makes iteration slower and creates large unrelated generated diffs, which is frustrating for contributors and reviewers.

## Rough Implementation Sketch

- Add a `compile-workflow` Make target that requires `WORKFLOW=` and compiles only the matching workflow source/lock file.
- Extend `scripts/dogfood.sh` with an optional workflow filter so sync can regenerate only `trigger-.yml` (including existing dogfood override and remediation-chain behavior).
- Keep `make compile` as the full-repo path; the new target is additive for focused edits.
- Document the new command in `gh-agent-workflows/DEVELOPING.md` next to the current `make compile` guidance.

## Why It Won't Be That Hard

This is a focused ergonomics improvement with a small implementation surface (Makefile target + optional filter in one script + docs). It reuses existing compile/sync logic rather than introducing a new build system path.

## Evidence

- `Makefile#L54-L63` lists current make targets and only exposes full `compile`.
- `Makefile#L167-L183` shows `compile` always runs full `sync` plus full compile/postprocess steps.
- `scripts/dogfood.sh#L55-L150` iterates all `gh-agent-workflows/*/example.yml`, regenerating all triggers every run.
- `gh-agent-workflows/DEVELOPING.md#L145-L154` documents `make compile` as the standard edit loop, with no targeted compile option.
- Recent PR flow is active and frequent (for example #1026, #1030), so tighter iteration loops would pay off immediately for maintainers.

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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/25314385825)

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