fix: improve config file discovery from target paths

[DaisukeYoda]

Summary

• Fix -c flag to accept direct file paths (not just directories)
• Fix config auto-discovery to search from target analysis paths, not just CWD
• Prioritize .pyscn.toml over pyproject.toml across parent directories
• Only use pyproject.toml if it contains [tool.pyscn] section

Problem

Previously, config file discovery had two issues:

1. The -c flag expected a directory path, not a file path. Specifying a config file directly (e.g., -c myconfig.toml) didn't work.

2. FindDefaultConfigFile only searched the current working directory. Config files in parent directories of the target weren't discovered.

For example, with this structure:

project/
├── .pyscn.toml          # pyscn config
└── subproject/
    ├── pyproject.toml   # subproject's own pyproject.toml
    └── src/

Running pyscn analyze project/subproject/src would find subproject/pyproject.toml instead of project/.pyscn.toml.

Solution

• LoadConfig now handles both file paths and directory paths
• FindConfigFileFromPath prioritizes .pyscn.toml across all parent directories before falling back to pyproject.toml
• pyproject.toml is only used if it contains [tool.pyscn] section
• analyze_usecase.go auto-discovers config from target paths when no explicit config is specified

Test plan

• Build succeeds
• All existing tests pass
• Manual testing with nested directory structures
• Verified -c flag works with direct file paths
• Verified config auto-discovery from target paths