Add symbols discovery command

[baksvell]

Summary

Adds the new livedoc symbols command for discovering reusable code_id values in a project.

Changes

• extracted shared source-code discovery into livedoc.core.discovery
• added livedoc symbols [path]
• added text and JSON output formats
• included symbol signatures, source files, and line numbers
• added duplicate code_id validation
• preserved .livedocignore, configuration, and --ignore handling
• documented the new command in the README and CLI help
• preserved compatibility with the existing documentation-check command

Examples

livedoc symbols .
livedoc symbols . --format json

Validation

• Ruff passed
• 77 tests passed
• Python, TypeScript/JavaScript, and Go discovery verified
• existing documentation freshness check passed
• existing CLI behavior remains compatible

Notes

The package version intentionally remains 0.1.8. It will be changed to 0.2.0 during release preparation after the planned 0.2.0 functionality is completed.