Add --map and --from flags for scoped repo serialization

[MedUnes]

Summary

• Add --map flag that produces a lightweight structural overview (hierarchical tree with token estimates, git last-modified annotations, and regex-based signature extraction for 10+ languages) instead of full file contents
• Add --from flag that scopes output to specific files or directories (comma-separated), with warnings for missing/ignored paths
• Both flags can be combined: --map --from produces a scoped structural map
• Default dewdrops . behavior is unchanged

Details

• The map mode enables a two-step workflow: paste the map into an LLM chat to get architectural awareness (~2-5% of full dump tokens), then use --from to provide only the files the LLM asks for.

• Signature extraction covers Go, Python, JS/TS, Rust, Java/Kotlin, Ruby, PHP, C/C++, SQL, and Shell via regex patterns, with a fallback (first 3 non-empty lines) for unknown file types. Token estimation uses len(bytes) / 4.

• Refactored Run() to accept a RunOptions struct. Extracted gitignore loading and path filtering into helpers. No new external dependencies.