feat: add database platform docs (Snowflake, Databricks, DuckDB, PostgreSQL, ClickHouse)

Context7 doesn't index Snowflake/Databricks platform docs (SQL reference,
DDL, functions). Neither platform exposes a documentation MCP server that
works without account credentials.

Solution: use webfetch for platform SQL docs alongside ctx7 for library
docs. Added curated URL mappings for Snowflake, Databricks, DuckDB,
PostgreSQL, ClickHouse, and BigQuery official documentation. Also added
Python connector library IDs (duckdb, psycopg, clickhouse-connect) and
dbt adapters (dbt-duckdb, dbt-clickhouse) to the Context7 mappings.

https://claude.ai/code/session_01NZPdvEHNXDcmhgJt9RLMu1

Author: claude

.opencode/skills/data-docs/SKILL.md

@@ -1,11 +1,12 @@
 ---
 name: data-docs
 description: >-
-  Fetch up-to-date, version-aware documentation for data engineering tools.
-  Use this skill when writing code that uses dbt, Airflow, Spark, Snowflake,
-  BigQuery, Databricks, Kafka, SQLAlchemy, Polars, or Great Expectations.
-  Activates for API lookups, configuration questions, code generation, or
-  debugging involving these data tools.
+  Fetch up-to-date, version-aware documentation for data engineering tools
+  and database platforms. Use this skill when writing code or SQL that uses
+  dbt, Airflow, Spark, Snowflake, BigQuery, Databricks, DuckDB, PostgreSQL,
+  ClickHouse, Kafka, SQLAlchemy, Polars, or Great Expectations. Activates
+  for API lookups, SQL syntax, configuration questions, code generation, or
+  debugging involving these data tools and platforms.
 ---
 
 # Data Engineering Documentation Lookup
@@ -23,15 +24,30 @@ Activate this skill when the user:
 - Works with PySpark transformations or Spark SQL
 - Uses Snowflake SQL, Snowpark, or the Snowflake Python connector
 - Uses BigQuery SQL or the Python client library
-- Works with Databricks SDK or notebook code
+- Works with Databricks SQL or the Python SDK
+- Writes DuckDB SQL or uses the DuckDB Python API
+- Writes PostgreSQL SQL, functions, or extensions
+- Works with ClickHouse SQL, engines, or functions
 - Writes Kafka producer/consumer code
 - Uses SQLAlchemy ORM or Core queries
 - Works with Polars DataFrame operations
 - Sets up Great Expectations data validation
-- Asks "how do I" questions about any data engineering library
-- Needs API references, method signatures, or configuration options
+- Asks "how do I" questions about any data engineering library or platform
+- Needs SQL syntax, API references, method signatures, or configuration options
 
-## How to Fetch Documentation
+## Documentation Sources
+
+This skill uses **two methods** depending on the type of documentation:
+
+1. **Context7 CLI** (`ctx7`) — For Python libraries and SDKs (dbt-core, Airflow,
+   PySpark, Snowpark, etc.). These have indexed documentation in Context7.
+2. **Web Fetch** (`webfetch`) — For database platform SQL documentation (Snowflake SQL,
+   BigQuery SQL, Databricks SQL, DuckDB, PostgreSQL, ClickHouse). These platforms
+   maintain official docs sites that can be fetched directly.
+
+Check `references/library-ids.md` for the full mapping of which method to use.
+
+## Method 1: Context7 CLI (for Python libraries/SDKs)
 
 ### Step 1: Identify the Library
 
@@ -69,30 +85,58 @@ Write **specific, detailed queries** for better results:
 - Good: `"How to create incremental models with merge strategy in dbt"`
 - Bad: `"incremental"`
 
-### Step 4: Use the Documentation
+## Method 2: Web Fetch (for database platform SQL docs)
+
+For Snowflake, BigQuery, Databricks, DuckDB, PostgreSQL, and ClickHouse
+platform documentation (SQL syntax, functions, DDL, configuration), use
+the `webfetch` tool to fetch specific documentation pages.
+
+### Step 1: Find the Right URL
+
+Check `references/library-ids.md` for the **Platform Documentation URLs**
+section. Each platform has a base URL and common page paths listed.
+
+### Step 2: Fetch the Documentation
+
+Use the `webfetch` tool with the specific documentation URL and a prompt
+describing what information to extract:
+
+```
+webfetch(url="https://docs.snowflake.com/en/sql-reference/sql/merge",
+         prompt="Extract the full MERGE syntax, parameters, and examples")
+```
+
+### Step 3: Use the Documentation
 
 - Answer using the fetched documentation, not training data
 - Include relevant code examples from the docs
-- Cite the library version when relevant
+- Cite the documentation URL for reference
 - If docs mention deprecations or breaking changes, highlight them
 
 ## Guidelines
 
-- Maximum 3 CLI calls per user question to avoid rate limits
-- Works without authentication; set `CONTEXT7_API_KEY` env var for higher rate limits
-- If a CLI call fails (network error, rate limit), fall back to training data
+- Maximum 3 CLI/webfetch calls per user question to avoid rate limits
+- Context7 works without authentication; set `CONTEXT7_API_KEY` for higher limits
+- If a call fails (network error, rate limit), fall back to training data
   and note that the docs could not be fetched
 - For dbt: always check `dbt_project.yml` for version and `packages.yml` for packages
 - For Python tools: check `requirements.txt` or `pyproject.toml` for pinned versions
 - When multiple libraries are relevant (e.g., dbt-core + dbt-snowflake), fetch docs
   for the most specific one first
+- For SQL platform docs, prefer the most specific page URL (e.g., the MERGE
+  statement page, not the general SQL reference index)
 
 ## Usage
 
 - `/data-docs How do I create an incremental model in dbt?`
 - `/data-docs What Airflow operators are available for BigQuery?`
 - `/data-docs How to use window functions in PySpark?`
 - `/data-docs Snowpark DataFrame API for joins`
-
-Use the bash tool to run `ctx7` CLI commands. Reference `library-ids.md` for
-pre-mapped library IDs to skip the resolution step.
+- `/data-docs Snowflake MERGE statement syntax`
+- `/data-docs DuckDB window functions`
+- `/data-docs PostgreSQL JSONB operators`
+- `/data-docs ClickHouse MergeTree engine settings`
+
+Use the bash tool to run `ctx7` CLI commands for libraries, and the `webfetch`
+tool for platform SQL documentation. Reference `library-ids.md` for the full
+mapping of tools, IDs, and URLs.