docs: reframe JJ as implementation detail in README

[timeleft--]

Summary

• Lead with "Git-native, curated memory" instead of "VCS-backed" in the opening
• Remove JJ from opening paragraphs and "Why" section — describe outcomes, not tools
• Rewrite Prerequisites to lead with colocate reassurance ("Your repo remains a standard Git repo")
• Add note to Manual Push section clarifying most users never need those commands

Consensus Review

• GPT 5.4 Pro (for): 8/10 messaging clarity, 8/10 adoption friction reduction — endorses direction
• Gemini 3.1 Pro (neutral): 10/10 — "masterful progressive disclosure", technically accurate framing
• Grok 4.1 Pro unavailable on OpenRouter

Manual follow-up needed

• Update GitHub About description (web UI): remove "Backed by Jujutsu VCS", add "Draft isolation, promotion gates, supersession chains"
• Add git-native GitHub topic (web UI)

Test plan

• All 618 tests pass (uv run pytest -v)
• Visual review of README rendering on GitHub

🤖 Generated with Claude Code