Add --since, -o, and review fixes for v0.3.0

[MedUnes]

Summary

• Add --since flag that 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 oversize output warning when output exceeds ~100K estimated tokens.
• Add markdown heading extraction (.md files now produce #-heading signatures instead of fallback).
• Pre-compile all signature regexes as package-level vars (was recompiling per file).
• Batch git mod-time lookups with 16 concurrent goroutines (was sequential per file).
• Wrap --map tree 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: what the
  changed files look like structurally (map with signatures), what changed (diff), and the full context where changes landed (content). Deleted files appear in the tree with [D] and (deleted) but are excluded from the content
  section.