feat: add AI Agent Identity scaffoldling

Description

Adds the AI Agent Identity scaffolding. This will the first in a small series of MRs to build the functionality for AI Agent Identity.

How has this been tested?

make build

# Test consent prompt
./bin/glab ai setup

# Prompts: "glab will configure hooks for AI agent governance. Do you wish to continue?"
# Answer No → prints "Setup cancelled."
# Answer Yes → installs hooks

# Verify hooks written correctly
cat ~/.claude/settings.json

# Test idempotency, run twice, hooks should not be duplicated
./bin/glab ai setup 
cat ~/.claude/settings.json

# Run unit test
go test gitlab.com/gitlab-org/cli/internal/commands/ai/setup -v

# Verify setup with doctor
./bin/glab ai doctor