[product-manager-impersonator] Configurable prompt-audit artifact directory

## Feature Idea

**Summary:** Add a configurable artifact/output directory for prompt-audit helper scripts and workflow instructions instead of hardcoding `/tmp` paths.

## Why a Customer Would Want This
Teams running these workflows in locked-down CI runners or local environments often have restricted or ephemeral `/tmp` semantics, which makes prompt-audit setup brittle and harder to reproduce. A configurable path improves reliability and makes investigation artifacts easier to retain.

## Rough Implementation Sketch
- Add a shared output-root input/env (for example `artifact-root` or `PROMPT_AUDIT_DIR`) and plumb it through prompt-audit setup commands.
- Update `scripts/extract-lockfile-prompts.sh` to default to that configured location rather than `/tmp/prompt-audit`.
- Update `scripts/fetch-workflow-logs.py` default output dir from `/tmp/gh-aw/logs` to the same configurable root (with CLI override preserved).
- Update `trigger-prompt-audit.yml` instructions to reference the configured directory path consistently.

## Why It Won't Be That Hard
The change is localized to two helper scripts plus one trigger workflow prompt/instructions. All needed extension points already exist (`--output-dir` in Python script and positional output-dir argument in shell script), so this is mostly wiring defaults and docs.

## Evidence
- `scripts/fetch-workflow-logs.py` hardcodes `/tmp/gh-aw/logs` as default in both usage text and argparse defaults (`L14`, `L140-L141`).
- `scripts/extract-lockfile-prompts.sh` hardcodes `/tmp/prompt-audit` as the default output directory (`L6`, `L15`).
- `.github/workflows/trigger-prompt-audit.yml` assumes extracted prompts are written to `/tmp/prompt-audit/` in assignment instructions (`L24`, `L28`).

## Duplicate Check
Searched open issues for equivalent requests and did not find an existing match:
- `repo:elastic/ai-github-actions is:issue is:open "prompt-audit" "output-dir"`
- `repo:elastic/ai-github-actions is:issue is:open "fetch-workflow-logs.py" "/tmp/gh-aw/logs"`
- `repo:elastic/ai-github-actions is:issue is:open "extract-lockfile-prompts.sh" "/tmp/prompt-audit"`

Closest related issue is `#987` (fail-closed dedupe mode for Scheduled Audit), which changes dedupe behavior and not artifact path configurability.

> [!NOTE]
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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/24830287728)

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