refactor(core-agent): move spark-command agent from stage-ui to core-agent (#2528)

## Summary

Move `spark-command` and `spark-notify` agents from
`packages/stage-ui/src/tools/character/orchestrator/` to
`packages/core-agent/src/agents/` for better separation of concerns and
reusability across packages.

## Changes

- **Relocate spark-command agent** — moved `spark-command-shared.ts`,
`spark-command.ts`, and `spark-command.test.ts` to
`packages/core-agent/src/agents/spark-command/`
- **Relocate spark-notify test** — moved `spark-notify.test.ts` to
`packages/core-agent/src/agents/spark-notify/`
- **Add barrel export** — created
`packages/core-agent/src/agents/spark-command/index.ts` with re-exports
- **Update package.json** — added `./agents/spark-command` entry in
`packages/core-agent/package.json`
- **Update build config** — added `src/agents/spark-command/index.ts` to
`tsdown.config.ts` entry list
- **Consolidate schemas** — merged duplicate
`sparkNotifyCommandGuidanceSchema` into `sparkCommandGuidanceSchema`
- **Add JSDoc** — added `@example` blocks to all normalize functions in
`schema.ts`
- **Update imports** — provider tests and `tool-resolver.ts` now import
from `@proj-airi/core-agent/agents/spark-command`
- **Delete legacy shims** — removed `stage-ui/src/tools/character/`
re-export layer

## Verification

```bash
pnpm -F @proj-airi/core-agent typecheck
pnpm -F @proj-airi/stage-ui typecheck
pnpm -F @proj-airi/core-agent exec vitest run
pnpm lint
```

packages/core-agent/package.json

@@ -23,6 +23,10 @@
     "./agents/spark-notify": {
       "types": "./dist/agents/spark-notify/index.d.mts",
       "default": "./dist/agents/spark-notify/index.mjs"
+    },
+    "./agents/spark-command": {
+      "types": "./dist/agents/spark-command/index.d.mts",
+      "default": "./dist/agents/spark-command/index.mjs"
     }
   },
   "main": "./dist/index.mjs",