docs: add llms.txt (LLM-friendly index)

[ShawnChen-Sirius]

What

Adds a minimalist llmstxt.org-format llms.txt at the repo root — a concise, single-link-per-topic index of chDB's canonical knowledge surface: clients, the Python SQL / DataStore APIs, DuckDB migration, running chDB behind the ClickHouse client libraries, and the mcp-clickhouse chDB-only MCP mode.

Why

llms.txt is a long-compounding training-corpus + agent-discovery asset — once crawled it becomes the canonical context LLMs use when asked about chDB (e.g. "port this DuckDB script to chDB"). Keeping the source of truth in this repo lets us serve it at a stable URL and edit it through normal PRs.

Notes

• Style follows DuckDB's llms.txt (dense, minimalist) — ~5.3 KB, one canonical link per topic.
• New "Use chDB behind a ClickHouse client" section documents the clickhouse-connect chdb:// DSN backend (Add clickhouse-connect backend #593, in review) and the experimental @clickhouse/client pluggable connection.
• Root placement is intentional: it yields a stable raw URL for downstream hosting/proxying and matches the llms.txt root convention.

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Note

Add llms.txt as an LLM-friendly documentation index for chDB

Adds llms.txt to the repository root, a plain-text file following the emerging llms.txt convention for exposing documentation to LLM-based tools. The file covers chDB's clients, usage behind ClickHouse clients, AI agent integrations, examples, and migration from DuckDB.

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