Hotfix: switch release-plz to manual trigger only

[RAprogramm]

Problem

After the Phase 0 merge of #3, the release-plz workflow ran on main and failed:

ERROR Failed to open PR

Caused by:
    0: received unexpected response
    1: Response body:
       {
         "message": "GitHub Actions is not permitted to create or approve pull requests.",
         "status": "403"
       }

Run: https://github.com/RAprogramm/RustManifest/actions/runs/25782380917

The default GITHUB_TOKEN is not permitted to create PRs in this repository, and CARGO_REGISTRY_TOKEN is intentionally not set — the project is at 0.0.0, untested end-to-end, and not ready to publish to crates.io.

This means every push to main will produce a red workflow run until publishing is actually enabled in a later phase.

Fix

Switch .github/workflows/release-plz.yml from push: branches: [main] to workflow_dispatch: only. This stops the automatic noise; the workflow can be run manually from the Actions tab if needed for testing, and flipped back to automatic when the project is actually ready to release.

No other changes. The workflow body, the release-plz.toml config, and the release.yml (tag-triggered binary release) all stay exactly as-is.

Out of scope

• Setting CARGO_REGISTRY_TOKEN — not until the project is ready to publish.
• Enabling repo-level "Allow GitHub Actions to create pull requests" — deferred to the same future moment.
• Tag-triggered binary release pipeline (release.yml) — already fires only on explicit v*.*.* tag pushes, no noise.

Acceptance criteria

• .github/workflows/release-plz.yml triggers exclusively on workflow_dispatch.
• reuse lint still 100% compliant.
• CI on the PR is green.
• After merge, no new automatic release-plz run appears on main.