Add toolchain dead-code scans

[alxxjohn]

Summary

• add opt-in quality.dead-code.toolchain coverage for Go, Rust, C++, Python, TypeScript, and JavaScript
• add trusted artifact/report ingestion for linker/symbol, bundler, and vulture-style evidence with conservative false-positive guards
• add repeatable CLI config overrides via -set key=value so agents can tune scans without writing temporary config files
• document artifact generation, reviewer guidance, and CLI override examples

Validation

• node --check internal/codeguard/checks/support/typescript_semantic_runner_core.js
• node --check internal/codeguard/checks/support/typescript_semantic_runner_dead_code_reports.js
• go test ./tests/cli -count=1
• go test ./tests/checks -run 'Test(Go|CPP|Rust|Python|Script)ToolchainDeadCode' -count=1
• go test ./... -count=1
• git diff --check
• make codeguard-ci: 6 pass, 0 warn, 0 fail, 0 findings, 2 suppressed