fix: harden validate CLI + scrub registry path leaks

[zenprocess]

Summary

Follow-up to #2 after a leak/CI audit. Three things:

1. Hardened `validate` CLI

• Accepts files OR directories (recursed for `.yaml`/`.yml`)
• Accepts multiple paths in one invocation
• `exists=True`/`file_okay`/`dir_okay` on the typer Argument — typos fail before the function body runs
• Always loud about results: prints "N/M valid" summary
• Exit codes: `0` all valid, `1` validation failure, `2` no .yaml/.yml found in given paths
• This kills the silent-literal-glob failure mode that hid the original CI breakage on `main`

2. validate-registry CI step collapses to one invocation

```yaml

• name: Validate all registry configs
  run: python -m servingcard validate registry/
  ```
  No more bash glob fragility.

3. Registry path leak scrub

4 YAMLs were publishing absolute `/home//models/...` paths in `serving.engine_args.model`. Replaced with `${MODEL_DIR}/...` convention:

• registry/deepseek-coder-v2-lite/gb10-fp8-baseline.yaml
• registry/devstral-small-24b/gb10-baseline.yaml
• registry/qwen3-coder/gb10-fp8-baseline.yaml
• registry/qwen3-coder/gb10-fp8-eagle3-spec3.yaml

All 6 registry cards still validate.

Test plan

• `pytest tests/test_validate.py` — 16/16 (4 new regression tests)
• `ruff check servingcard/ tests/` — clean
• `python -m servingcard validate registry/` — 6/6 valid
• CI test (3.10/3.11/3.12) green
• CI validate-registry green

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
packages/python/servingcard/cli.py	80.00%	4 Missing ⚠️

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