docs: update docs/architecture/workflows.md and release-process.md for Pre-Release-As trailer feature

## Component

Documentation

## Bug Description

The `Pre-Release-As:` commit trailer feature was added to `release-prerelease-pr.yml` in commit `310c68c` (PR #1565) but is not documented in either of the two mapped documentation files:

- `docs/architecture/workflows.md` — describes `release-prerelease-pr.yml` only as "Pre-release companion PR management" with no mention of the trailer override capability
- `docs/contributing/release-process.md` — documents release-please's `Release-As:` mechanism but has no equivalent section for the pre-release `Pre-Release-As:` trailer

The implementation in `.github/workflows/release-prerelease-pr.yml` (lines 81–87) honors a `Pre-Release-As:` commit trailer to override the computed pre-release version, mirroring the stable pipeline's `Release-As:` trailer behavior.

## Expected Behavior

- `docs/architecture/workflows.md` should document the `Pre-Release-As:` trailer as a version override mechanism for the `release-prerelease-pr.yml` workflow, alongside the existing description of that workflow.
- `docs/contributing/release-process.md` should include a note (similar to the `Release-As:` documentation) explaining that maintainers can add `Pre-Release-As: X.Y.Z` as a commit trailer to force a specific pre-release version.

## Steps to Reproduce

1. Review commit `310c68c` — "feat(workflows): add Pre-Release-As trailer override to pre-release w..."
2. Check `.github/workflows/release-prerelease-pr.yml` lines 81–87 for the implementation
3. Search `docs/architecture/workflows.md` and `docs/contributing/release-process.md` for `Pre-Release-As` — no results found

## Additional Context

- Changed workflow file: `.github/workflows/release-prerelease-pr.yml`
- Documentation files to update: `docs/architecture/workflows.md`, `docs/contributing/release-process.md`
- The feature mirrors `release-please`'s `Release-As:` trailer, so the documentation pattern from that feature is a good reference

> Generated by [Documentation Update Check](https://github.com/microsoft/hve-core/actions/runs/25843212739/agentic_workflow) · ● 1.2M · [◷](https://github.com/search?q=repo%3Amicrosoft%2Fhve-core+is%3Aissue+%22gh-aw-workflow-call-id%3A+microsoft%2Fhve-core%2Fdoc-update-check%22&type=issues)

Read .github/workflows/release-prerelease-pr.yml lines 81–87 to confirm how the Pre-Release-As: trailer overrides the computed version. Then compare the existing Release-As: documentation in docs/contributing/release-process.md and the workflow description in docs/architecture/workflows.md. Done means both files explain the trailer and its X.Y.Z version override behavior.

Written by the indexing model from the issue text.