refactor: unify CLI to `macdoc convert --to` pattern (textutil-compatible)

[kiki830621]

Summary

Migrate all conversion subcommands to a unified macdoc convert --to <format> entry point, following macOS textutil -convert <fmt> patterns.

Current State

macdoc word to-md file.docx
macdoc html to-md file.html
macdoc md to-html file.md
macdoc srt to-html file.srt
macdoc bib to-html file.bib
macdoc bib to-md file.bib
macdoc bib to-json file.bib

Each source format is a separate subcommand with its own to-* children.

Target State

macdoc convert --to md file.docx
macdoc convert --to md file.html
macdoc convert --to html file.md
macdoc convert --to html file.srt
macdoc convert --to html file.bib --style apa
macdoc convert --to md file.bib --style apa
macdoc convert --to json file.bib --style apa
macdoc convert --to tokens file.md

Design Principles (from .claude/rules/cli-textutil-compat.md)

• Action first: convert --to <format>
• File last: always the trailing argument
• Auto-detect input format from file extension (.docx → Word, .html → HTML, .bib → BibLaTeX, etc.)
• Long flags: --to, --output, --style (swift-argument-parser convention)
• Non-conversion subcommands unchanged: pdf init/status/..., config ai detect/...

Routing Logic

Input extension	--to	Package
.docx	md	word-to-md-swift
.html	md	html-to-md-swift
.md	html	md-to-html-swift
.srt	html	srt-to-html-swift
.bib + --style apa	html	bib-apa-to-html-swift
.bib + --style apa	md	bib-apa-to-md-swift
.bib + --style apa	json	bib-apa-to-json-swift
any text	tokens	token-counter-swift (issue #20)

Backward Compatibility

Old subcommands (word to-md, html to-md, etc.) kept as aliases during migration, with deprecation warning. Remove in next major version.

Options

macdoc convert --to <format> [options] <file>

Options:
  --to <format>       Target format (md, html, json, latex, tokens)
  --output <path>     Output file path (default: stdout or same dir with new extension)
  --stdout            Force output to stdout
  --style <style>     Citation/formatting style (e.g. apa) — for bib conversions
  --model <model>     Model for token counting (gpt-4o, claude-sonnet)
  --hard-breaks       Treat soft breaks as hard breaks (md→html)
  --full              Output full HTML document instead of fragment

References

• .claude/rules/cli-textutil-compat.md
• macOS textutil -convert syntax
• swift-argument-parser conventions