docs: refocus Lovcode as AI conversation search (v0.40 rewrite kickoff)

Rewrite README to reflect the new product direction: a single-purpose
AI conversation search tool with four surfaces (CLI, MCP, Tauri desktop
with floating window, Web UI) backed by one Rust core crate and an
adapter-based ingest layer (Claude Code, Codex, Claude Desktop, ChatGPT,
Gemini, etc.).

The legacy v0.39 codebase (Workbench / Skills / Marketplace / Lab /
MaaS) is preserved on the legacy/v0.39-workbench branch.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: MarkShawn2020