Add dependency override governance gates

[yanziwei]

Summary

Adds dependency override/replacement governance to dependency-scanning.

Changes

• triggers the skill when override or replacement controls are present;
• adds an Override and Replacement Governance section for npm, Yarn, pnpm, Go, Cargo, Maven/Gradle, and Python constraints;
• adds DEP-OVERRIDE-01 through DEP-OVERRIDE-06 evidence gates;
• adds false-positive boundaries for governed fixed-version overrides and development-only workspace replacements;
• extends the output template with an override/replacement review table;
• updates the procedure and references.

Related issue

Fixes #1649.

Validation

• git diff --check
• Markdown fence balance check
• Required marker checks for DEP-OVERRIDE-01, override table, Go replace, and Cargo patch
• Added-line ASCII check

Bounty

Target: Improver Moderate ($100) if accepted. Payment details can be provided privately after maintainer acceptance.