feat(checks): add claude-opus-5 to known model list

[protosphinx]

Why

claude-opus-5 shipped as part of the Claude 5 family alongside claude-sonnet-5 and claude-fable-5. It was missing from KNOWN_CLAUDE_MODELS in checks.ts, so any skill that sets model: claude-opus-5 receives a spurious model-unknown warning. This closes that gap.

What

• Add "claude-opus-5" to KNOWN_CLAUDE_MODELS in src/checks.ts.
• Add test "does not warn on claude-opus-5" in test/checks.test.ts, following the same pattern as the existing per-model tests.

Tests

• One new test in test/checks.test.ts: verifies that runChecks emits no model-unknown diagnostic when model: claude-opus-5 is set.
• All 89 existing tests continue to pass unmodified.

Self-merge gate

• all CI checks pass
• LOC delta < 250 (9 lines added, 0 removed)
• no public-API surface change (src/index.ts not modified)
• no runtime-dependency additions
• no workflow file changes
• tests added or extended

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Generated by Claude Code

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