feat(rust): port config validate to native Rust (Phase 1.3) (#1282)

First native command. Exercises every piece of the foundations
work so far — HTTP fetch, JSON-schema validation, output
discipline, typed exit codes — and proves the shim-fallback
dispatch pattern.

## What ports natively

``mergify config validate [-f PATH]``:

1. Resolves the config file (explicit ``--config-file`` or the
   first of ``.mergify.yml``, ``.mergify/config.yml``,
   ``.github/mergify.yml``).
2. Parses it as YAML via ``serde_norway``. Empty files are treated
   as an empty mapping; non-mapping top-level values are rejected.
3. Fetches the published schema from
   ``https://docs.mergify.com/mergify-configuration-schema.json``
   via the Phase 1.2b HTTP client (30s timeout, retry on 5xx).
4. Validates with the ``jsonschema`` crate (draft-07).
5. Emits a human-readable success line or a sorted, path-prefixed
   per-error list via ``Output::emit``.

Exit codes: success → 0, missing / unparseable / schema violations
→ 8 (``CONFIGURATION_ERROR``), schema-fetch failure → 6
(``MERGIFY_API_ERROR``).

## Dispatch

``mergify-cli/src/main.rs`` inspects argv for a ``config validate``
pair; if present, it parses with clap and runs the native path.
Everything else — including ``config simulate``, every ``stack``
command, the global ``--version`` / ``--help``, plus any invalid
invocation — falls through to the Python shim unchanged. Clap is
loaded only when a native match is suspected, so unknown flags on
un-ported commands still produce Python's click error messages
verbatim.

## Smoke-tested against the real docs CDN

Missing config → exit 8. Valid config → exit 0. Invalid config →
detailed error list + summary, exit 8. ``mergify --version`` still
falls through to Python.

## Test coverage

11 unit tests in ``mergify-config``:

- ``resolve_config_path`` with explicit, default, and missing paths
- ``load_yaml`` happy path, scalar rejection, malformed YAML,
  empty-file-is-empty-mapping
- ``validate_against_schema`` happy path + error aggregation
- ``emit_result`` for success and failure rendering

## Binary size

2.7 MB → 8.0 MB. The jump is jsonschema pulling fancy-regex + the
referencing crate's schema resolver. Still well under the 15 MB
design target; we can trim with features later if needed.

## Follow-up

- 1.3b: port ``config simulate``
- 1.4+: ``ci`` commands (first CI-facing wins for the port story)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jd