refactor: migrate conventional-title config from string to dict

[myakove]

Summary

Migrate the conventional-title setting under ai-features from a string format to a dict format for consistency with resolve-cherry-pick-conflicts-with-ai.

Problem / Motivation

The conventional-title setting under ai-features is currently a string ("true", "false", "fix"). For consistency with resolve-cherry-pick-conflicts-with-ai (which uses a dict with enabled and timeout-minutes), it should also be migrated to a dict format.

Current Format

ai-features:
  conventional-title: "true"  # or "false" or "fix"

Proposed Format

ai-features:
  conventional-title:
    enabled: true
    mode: "suggest"  # or "fix"
    timeout-minutes: 10

Requirements

1. Update the configuration schema (webhook_server/config/schema.yaml) to support the new dict format for conventional-title
2. Update all code references that read conventional-title as a string to handle the new dict format
3. Update config examples to reflect the new format
4. Ensure backward compatibility for user-facing configuration (support old string format or provide migration)

Deliverables

• Code changes
• Update README.md (if applicable)
• Update CLAUDE.md (if codebase structure changes)
• Add/update tests (if repo has tests)
• Update schema validation
• Update config examples

Notes

• This is a follow-up to feat: AI-powered conflict resolution for auto cherry-pick #1022 which established the dict pattern for resolve-cherry-pick-conflicts-with-ai
• Schema, config examples, and all code references to conventional-title need updating
• Per project policy, backward compatibility IS required for user-facing configuration files