[Task]: Add a local 'make validation' target to run the suite against box

[michael-duren]

What needs to be done

Add a make validation (and/or scripts/validation.sh) target that builds
box, points the integrated runtime-tools suite at it, and runs the validation
tests locally with one command.

The target should:

• Build the box binary (reuse existing make build wiring).
• Run the runtime-tools validation suite with RUNTIME set to the built binary.
• Surface results as readable output (TAP / pass-fail summary).
• Allow running a single test or a subset for focused debugging.
• Not fail the developer's whole build when individual tests fail (since the
  suite is expected to be partially red during active development).

Acceptance criteria

• make validation builds box and runs the suite in one command
• Results are printed as a clear pass/fail summary
• A way to run a single/subset of tests is documented (e.g. make validation TEST=...)
• The target is documented in the README "Development" section
• Running it produces a real (possibly partially-failing) result against current box

Parent / tracking issue

Part of the OCI runtime validation epic.

Blocked by

• Integrate the runtime-tools validation suite into the repo

Rough size

S — a few hours