Migrate content system to git-warp native API

[flyingrobots]

Summary

Replace the shell-out content-on-node implementation (execFileSync to git hash-object / git cat-file) with git-warp's native content API (patch.attachContent(), graph.getContent(), graph.getContentOid()).

Motivation

• git-warp v11.5.0 ships a native content API with GC protection, async persistence, and no subprocess overhead
• The current src/content.js shells out to git directly — redundant plumbing
• Clean break, major version bump to 4.0.0

Property Key Change

Old	New
_content.sha (string OID)	_content (WARP's CONTENT_PROPERTY_KEY)
_content.mime	unchanged
_content.size	unchanged

What Gets Removed

• execFileSync calls in content.js
• PREFIX, KEYS, SHA_RE, assertValidSha() helpers
• Integrity re-hash in readContent() (git CAS guarantees integrity)
• cwd parameter on writeContent and readContent

Files Modified

• src/content.js — rewrite internals
• src/cli/commands.js — drop cwd from 2 call sites
• test/content.test.js — update calls and property keys, remove integrity test
• package.json — bump to 4.0.0
• ROADMAP.md — update property reference