Feature: warn when workflow YAML validates but would fail at runtime (schema vs runner gap)

### Summary

Extension validates workflow YAML against the schema, but many mistakes only surface after `git push` triggers a runner:

- `runs-on:` referencing a self-hosted label that has no online runner
- `uses: owner/repo@ref` where `ref` no longer resolves (deleted tag/branch, moved SHA)
- `secrets: inherit` on a reusable workflow whose caller does not actually inherit
- `permissions:` narrower than what a step needs (e.g. `contents: read` + a step that pushes)
- `if:` expression referencing a context that is empty for the trigger (e.g. `github.event.pull_request.*` on `push`)

Each case validates green locally, then burns a runner minute and a red X on the PR.

### Proposal

A "runtime-plausibility" pass, opt-in (`github-actions.runtimeChecks.enabled`), that runs alongside schema validation and surfaces `Information`-level diagnostics for:

1. Unresolvable `uses:` refs (HEAD probe via the authenticated session already used for the API tree)
2. `runs-on:` labels not present in the repo's runner list
3. `permissions:` narrower than the union of permissions declared by any resolvable action's `action.yml`
4. Context references that are empty for the declared `on:` triggers

None block save; all are dismissible. Runs on open + on save, cached by workflow-file hash.

### Why not `act` / nektos

`act` runs the whole workflow in Docker; this is a static, seconds-scale lint. Complementary, not overlapping.

### Related

- #593 (commit-pinned actions reported unresolved) - same "static analysis of `uses:`" surface
- #609 (false-positive missing-required-inputs) - related schema-vs-runtime gap in the other direction

Start by tracing the existing workflow YAML schema-validation entry point and the authenticated session already used for the API tree. Review how action.yml files and workflow triggers are resolved, then determine how the four opt-in runtime checks and workflow-file-hash cache fit alongside validation. Done means dismissible Information diagnostics run on open and save without blocking saves.

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