feat(Add --since, -o, --version, map extension filter)

[MedUnes]

Summary

• Add --since flag: produces a composite review document (map of changed files with [M]/[A]/[D] status + full unified diff + complete content of changed files). Auto-names output as dewdrops_since_.md. Mutually
  exclusive with --map/--from.
• Add -o flag for custom output file path in all modes.
• Add --version flag, with version injected via goreleaser ldflags at release time (dev for local builds).
• Change --map from boolean to optional-value flag: --map (supported languages only), --map=go,py (specific extensions), --map=any (all text files). Eliminates noise from config files (.yaml, .json, .env) in default map
  output.
• Add oversize output warning when output exceeds ~100K estimated tokens.
• Add markdown heading extraction for .md files (headings instead of first-3-lines fallback).
• Pre-compile all signature regexes as package-level vars instead of per-file compilation.
• Batch git mod-time lookups with 16 concurrent goroutines instead of sequential per-file.
• Wrap --map tree output in a fenced ```text block for reliable copy-paste.

Details

• The --since flag is the flagship feature. It enables a workflow where the user runs dewdrops --since main ., pastes the output into an LLM, and asks "Review these changes." The output gives the LLM three views: structural map
  with change status, unified diff, and full content of changed files. Deleted files appear in the tree with [D] but are excluded from the content section.

• The --map extension filter uses Go's IsBoolFlag() interface so --map works as a bare flag (supported extensions only) while --map=value passes a filter. Default supported set: .go, .py, .js/.ts/.jsx/.tsx, .rs, .java/.kt, .rb,
  .php, .c/.h/.cpp/.hpp, .sh/.bash, .sql, .md. Config/data files excluded from default to avoid first-3-lines fallback noise.