refactor: use rule-based frontend diagnostics

[GRAMMAC1]

Summary

• Replace framework-specific default plugins with three frontend evidence producers: compiler, runtime, and dependency resolution.
• Add a unified FailureEvidence model with reducer/ranker logic so the lead diagnosis is selected from evidence rather than plugin registration order.
• Move TypeScript, React, Vue, Nuxt, Svelte, Vite, dependency, and runtime signals into rule-based producers.
• Keep the generic plugin as a low-priority fallback evidence source.
• Update tests around evidence producers and add coverage that reversed plugin order keeps the same lead diagnosis.
• Refresh the MCP benchmark report after the refactor.

Benchmark

• Cases: 7
• Signal matches: 21/26
• Average reduction: 56.9%
• Average total MCP ratio: 43.1%
• The previous short-log pressure case svelte-vite-bindable-build-failure now reports a positive 39.8% reduction.

Validation

• pnpm verify
• pnpm benchmark:mcp