fix(skills): add Blacksmith Testbox route

[andrei-hasna]

Why

The Codewith runtime policy pointed Rust builds at a nonexistent generic remote-sandbox-build skill, while the installed fallback routed to unrelated AWS Fargate tooling. The repository already owns a working Blacksmith Testbox workflow, so agents could not reliably discover or use the supported remote-build lane.

What changed

• add a canonical blacksmith-testbox skill owned by this repository
• route codex-rs remote builds through .github/workflows/blacksmith-testbox.yml
• document the separate interactive Testbox CLI lane
• explicitly reject generic Sandbox, Fargate, E2B, and Daytona as Testbox substitutes
• add focused contract tests for the skill and workflow interface

Verification

• python3 .codewith/skills/blacksmith-testbox/tests/test_skill_contract.py — Ran 5 tests; OK
• git diff --check origin/main...HEAD — exit 0
• staged/candidate secret scan before both commits — findingCount: 0; truncated: false
• remote branch readback — b00d2db059fa87ab83a8e6e704a192f4e3c813f8

Pending gate

Fresh Codewith and Claude installation/readback is not claimed. The supported installer cannot currently install this private exact-head source: download returns HTTP 404, then clone fallback reuses a partial non-empty destination. The transaction restored all three prior installed directories. Codewith task 7c30e3e2-c447-4898-8aa7-fb69e2a98941 owns that installer defect; this PR remains draft until the fresh-install gate and independent adversarial review pass.

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