[product-manager-impersonator] Configurable agent timeout budgets

## Feature Idea

**Summary:** Add an optional workflow-call timeout input so repository operators can tune agent runtime budgets per installed workflow instead of accepting the hard-coded 30/60/90 minute defaults.

## Why a Customer Would Want This
Teams running AI workflows have different tolerance for latency and spend depending on the job. A PR comment helper may need to fail fast, while scheduled audits can justify a longer window. Today an operator who wants a shorter or longer budget has to fork or copy the reusable workflow instead of adjusting the trigger YAML they already own.

## Rough Implementation Sketch
- Add a standard optional `timeout-minutes` (or `max-runtime-minutes`) `workflow_call` input to the core reusable workflows, starting with the five recommended workflows.
- Replace hard-coded top-level `timeout-minutes` values with that input while preserving the current default per workflow.
- Document the input in the relevant `gh-agent-workflows//README.md` files and show one commented trigger example value.
- Add the convention to `gh-agent-workflows/DEVELOPING.md` next to the existing standard workflow-call inputs.

## Why It Won't Be That Hard
The timeout is already centralized as a single frontmatter field in each workflow, and workflow inputs already follow a repeated pattern. The first iteration can be limited to the recommended core workflows, preserving existing defaults while giving operators an override in their caller workflow.

## Evidence
- `README.md:23-26` identifies the five recommended starter workflows that would be the natural first rollout surface.
- `.github/workflows/gh-aw-pr-review.md:26-72` defines workflow-call inputs, but `.github/workflows/gh-aw-pr-review.md:93` hard-codes `timeout-minutes: 90`.
- `.github/workflows/gh-aw-product-manager-impersonator.md:22-68` follows the same input pattern, while `.github/workflows/gh-aw-product-manager-impersonator.md:93` hard-codes `timeout-minutes: 90`.
- `gh-agent-workflows/DEVELOPING.md:164-195` documents the standard input convention for reusable workflows, so this has an obvious place to document the new operator control.

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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/28936427208)

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Start with README.md:23-26 to identify the five recommended workflows, then compare their workflow-call inputs and hard-coded timeout values, including .github/workflows/gh-aw-pr-review.md and .github/workflows/gh-aw-product-manager-impersonator.md. Read gh-agent-workflows/DEVELOPING.md:164-195 for the input convention. Done means the recommended workflows preserve their defaults while exposing the optional override, with corresponding README examples and convention documentation.

Written by the indexing model from the issue text.