Proposal: global `search` command across conversations, contacts, messages, and articles

## Summary

I'd like to add a top-level `chatwoot search ""` command that returns matches across **conversations, contacts, messages, and help-center articles** in one call, with `--only=` to restrict the output. This complements the existing per-resource search (`contacts --search`, `convs --query`) with a single cross-resource entry point.

## Endpoint

It would call the account-scoped dashboard search endpoint:

```
GET /api/v1/accounts/{account_id}/search # all four buckets
```

This is what the Chatwoot dashboard's global search uses (`Api::V1::Accounts::SearchController`). Two questions before I open the PR:

1. **Is `/search` a supported API surface** for the CLI to depend on? It isn't in the published OpenAPI spec (`internal/sdk/testdata/application_swagger.json` only lists `/contacts/search`), so I'd cover it with `httptest`-based tests rather than the swagger contract harness — matching the existing `internal/cmd/*_test.go`. Happy to add the path to the bundled spec instead if you'd prefer to treat it as official.
2. **Top-level command vs. flag?** A new top-level `search` reads naturally for cross-bucket search, but it overlaps conceptually with `contacts --search` / `convs --query`. Would you prefer a top-level command, or a different shape?

## Follow-up (separate PR)

A second, optional PR adds date-range filtering (`--after` / `--before` / `--on`, `--tz`) mapping to `since` / `until` query params on `/search`. Heads-up: those params are also dashboard-internal (not in the published spec) and only take effect when the account has the `advanced_search` feature — so it's cleanly separable and easy to defer.

Both PRs are ready to open once you confirm the direction. Thanks!

Start by reviewing the existing per-resource search commands and internal/cmd/*_test.go, then inspect the dashboard search endpoint and internal/sdk/testdata/application_swagger.json. Resolve whether /search is a supported API surface and whether this should be a top-level command or a flag before implementing. Done means the direction is agreed and the cross-resource command is covered by httptest-based tests.

Written by the indexing model from the issue text.