ci(snap): make snapcraft workflow iterable and add adopt-info

[jmelosegui]

Summary

Makes the snap workflow runnable on any branch and clears the next schema validation error so v0.1.1's snap build path can finally make it past validation.

Workflow triggers

• Add `workflow_dispatch` so we can run the snap workflow on demand.
• Add `pull_request` filtered to `snap/**` and the workflow file itself, so future PRs that touch the snap config get automatic feedback.
• Tag-push trigger remains; `snapcraft upload` is now gated on `refs/tags/v*` so PR and dispatch builds stop at `Build snap` and never touch the snap store.
• `SNAP_VERSION` falls back to `0.0.0-dev` outside tag pushes so the version string is always valid.

`snap/snapcraft.yaml`: add `adopt-info: git-sweep`

The v0.1.1 release run failed validation with:

```
has-base.core22
Value error, Required field 'version' is not set and 'adopt-info' not used.
```

Snapcraft now requires an explicit `adopt-info` reference whenever the version is set inside an `override-build` step via `craftctl set version=...`. Pointing it at the existing `git-sweep` part satisfies the validator without changing how the version is computed.

Test plan

• The `snapcraft` job on this PR runs (because `snap/**` and the workflow file are touched) and reaches at least the `Build snap` step without a schema error.
• After merge, `gh workflow run snapcraft.yml --ref main` runs cleanly.
• On the next `vX.Y.Z` tag push, the `Upload to Snap Store` step fires (gated on `refs/tags/v*`).

[jmelosegui]

Superseded -- removing snap distribution entirely instead of fixing it. The script-based install on Linux is sufficient for now and a snap was not pulling its weight.