Publish a GitHub Action that validates dotagent manifests in CI

As more people adopt dotagent and version their agents in git (the dotfiles use-case), CI needs to catch manifest mistakes before they hit production. Today the only way to validate is to install dotagent locally and run `dotagent doctor`.

A reusable GitHub Action would let anyone validate manifests on every PR without setting up Rust.

## Proposal

Publish a GitHub Action at `avelino/dotagent-validate-action` (or as a sub-action of this repo) that:

1. Downloads the latest released dotagent binary (or a pinned version).
2. Runs `dotagent doctor` against the repo's agents.
3. Surfaces errors and warnings as GitHub annotations on the PR.

Usage:

```yaml
- uses: avelino/dotagent/validate-action@v1
with:
root: ./agents
version: latest
fail-on: error # error | warning
```

## Acceptance criteria

- [ ] Action lives in `.github/actions/validate/` (or a sibling repo, implementer's call).
- [ ] Pinned to a release version by default; `latest` is opt-in.
- [ ] Cross-platform: works on `ubuntu-latest` and `macos-latest` runners.
- [ ] Errors become PR annotations on the relevant manifest line where possible.
- [ ] Documented in [`README.md`](README.md) Install section.
- [ ] Smoke-tested with a dummy bad manifest in a workflow.

## Where to start

- `crates/dotagent/src/commands/mod.rs::doctor` — already produces structured output.
- GitHub's [problem matcher format](https://github.com/actions/toolkit/blob/main/docs/problem-matchers.md) for surfacing errors as annotations.

## Non-goals

- Auto-fix mode. Validation only.
- Running agents in CI. Just static manifest checks + plugin discovery.

Start with crates/dotagent/src/commands/mod.rs::doctor to understand its structured output, then review GitHub's problem matcher format. Implement the action in .github/actions/validate/ or a sibling repository, covering pinned versions, cross-platform runners, annotations, and the root and fail-on inputs. Document it in README.md and smoke-test it with a workflow using a dummy bad manifest.

Written by the indexing model from the issue text.