ci: run release on pushes to main via fixed publish.yaml (#472)

## Summary

Make module releases run on every push to `main`, replacing the two
broken release workflows with a single fixed one.

Both prior workflows were broken against the current reusable
`PowerShellOrg/.github/.github/workflows/powershell-release.yml@main`:

- **`publish.yaml`** (push-to-`main`) startup-failed on every push to
`main` — it passed an `isPrerelease` input that #468 removed.
- **`release.yml`** (tag-triggered) never ran on merges and was itself
broken: it passed a `module-name` input the reusable workflow does not
declare, and a non-existent `PSGALLERY_API_KEY` secret.

### Approach

Rename `release.yml` → `publish.yaml` and fix it. The reusable workflow
is designed for the push-to-`main` model: its `check_version` job
compares the manifest `ModuleVersion` against PSGallery and gates both
`create_release` and `publish` on `(version_bumped || force)`. So
running it on every push to `main` only actually tags/publishes when the
manifest version is new — a normal merge that doesn't bump the version
is a no-op.

### Changes

- Trigger on `push: branches: [main]` (+ `workflow_dispatch` for
manual/`force` re-runs) instead of `push: tags: ['v*']`.
- Drop the `module-name: Plaster` input — not declared by the reusable
workflow (it derives the module name itself); passing it caused a
startup failure.
- Pass the secret as `PS_GALLERY_KEY` (the reusable's required secret
name), sourced from the repo's existing `PS_GALLERY_KEY` secret.
- Remove the duplicate/broken `release.yml`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

chore(release): 2.1.2 (#466)