Add CI validation for schema keyword drift

Action schemas and their `.keywords.json` files can drift because keyword refresh is manual and `sourceHash` is not checked. Add a deterministic CI check that compares each configured schema’s active actions with its keyword entries and reports missing or extra actions. When validation fails, print the appropriate `@collision keywords backfill --llm ` command. LLM regeneration should remain local, not run in CI.

A scan found existing drift in six other files:
| Keyword file | Missing actions |
| --- | --- |
| `codeActionsSchema.keywords.json` | 7 actions |
| `editorCodeActionsSchema.keywords.json` | `launchCopilotChat` |
| `desktop/actionsSchema.keywords.json` | `RestartService` |
| `github-cliSchema.keywords.json` | `prMergedStatus`, `myPullRequests` |
| `listSchema.keywords.json` | `deleteList` |
| `scriptActions.keywords.json` | 5 PowerShell flow actions |