Add incremental scanning and configuration file support

[nobuo-miura]

Overview

Add incremental scanning options and repository-level configuration support so SecretLens can scan only the necessary changes in local development and CI environments.

Tasks

• Add --staged and --source=staged for scanning staged changes
• Add --source=worktree for scanning uncommitted changes to tracked files
• Add --since for incremental history scans
• Add --commit-range for scanning a specific commit range
• Support .secretlens.yml and .secretlens.yaml
• Ensure explicitly passed CLI flags take precedence over configuration values
• Add global exclusion patterns through exclude
• Resolve relative paths from the configuration file’s directory
• Prevent auto-detected configurations from controlling the output path
• Exclude Slack webhook URLs from configuration files
• Support worktree scans before the first commit
• Return [] instead of null when JSON output contains no findings
• Update the English and Japanese documentation
• Add tests for the new behavior