Migrate gemini mutator from deprecated generative-ai-go to google.golang.org/genai

[batmac]

Context

github.com/google/generative-ai-go is deprecated and frozen upstream (Google's replacement is google.golang.org/genai). Because it is frozen, it pins an old cloud.google.com/go/ai API: bumping go/ai to v0.20.0 breaks its compilation (genai/files.go: VideoMetadata vs VideoFileMetadata proto mismatch), which is what made the first "go indirect dependencies" Renovate batch (#1158) fail CI.

As a stopgap, .github/renovate.json excludes cloud.google.com/go/ai from updates (see #1161 and the rule's description).

Task

• Port the gemini mutator (pkg/mutators/single/, uses github.com/google/generative-ai-go/genai) to google.golang.org/genai
• Drop generative-ai-go from go.mod
• Remove the cloud.google.com/go/ai exclusion rule from .github/renovate.json so the indirect dependency flows normally again

Notes

• The stopgap is stable: nothing breaks while this waits, we just don't get go/ai updates
• Deliberately deferred by the maintainer until the rest of the automation overhaul (weekly grouped renovate batches, digest pinning, least-privilege tokens — PRs chore(renovate): automerge github-actions majors without dashboard approval #1148–ci: least-privilege GITHUB_TOKEN permissions for all workflows #1156) has proven itself in normal operation

🤖 Issue drafted with Claude Code