docs: rewrite README.md to reflect current codebase state

[Copilot]

README was outdated — missing Linux/macOS instructions, undocumented commands/flags, an inaccurate file layout, and no coverage of environment variables.

Changes

Accuracy

• Requirements table clarifies Claude Code is only needed for host-native mode; VM mode runs it inside the guest
• File layout updated to match actual repo structure (scripts/adapters/catalog.json, state/vm/, state/tmp/, runtime/linux/vm-manifest.json, runtime/linux/cloud-init/, profiles/defaults/, profiles/local/)
• Linux/macOS behavior documented: linux-portable mode on non-Windows runs host-native with isolated portable auth (no VM)

Coverage gaps filled

• Commands table now includes --sync-back, --no-sync-back, --force (bootstrap), --mode (run), and --resume usage example
• Dedicated Authentication section covering OAuth credential path (state/auth/claude/host/) and API key mode
• New Environment Variables table documenting all ANTHROPIC_* and PCODER_* vars
• WHPX → TCG acceleration fallback noted in Portability Notes

Structure

• Quick Start split into Windows and Linux/macOS tabs
• Requirements converted from bullet list to table
• VM mode note surfaced with context at the point of introduction rather than buried after the requirements table

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.