feat: add data-platform plugin (v4.0.0)

Add new data-platform plugin for data engineering workflows with:

MCP Server (32 tools):
- pandas operations (14 tools): read_csv, read_parquet, read_json,
  to_csv, to_parquet, describe, head, tail, filter, select, groupby,
  join, list_data, drop_data
- PostgreSQL/PostGIS (10 tools): pg_connect, pg_query, pg_execute,
  pg_tables, pg_columns, pg_schemas, st_tables, st_geometry_type,
  st_srid, st_extent
- dbt integration (8 tools): dbt_parse, dbt_run, dbt_test, dbt_build,
  dbt_compile, dbt_ls, dbt_docs_generate, dbt_lineage

Plugin Features:
- Arrow IPC data_ref system for DataFrame persistence across tool calls
- Pre-execution validation for dbt with `dbt parse`
- SessionStart hook for PostgreSQL connectivity check (non-blocking)
- Hybrid configuration (system ~/.config/claude/postgres.env + project .env)
- Memory management with 100k row limit and chunking support

Commands: /initial-setup, /ingest, /profile, /schema, /explain, /lineage, /run
Agents: data-ingestion, data-analysis

Test suite: 71 tests covering config, data store, pandas, postgres, dbt tools

Addresses data workflow issues from personal-portfolio project:
- Lost data after multiple interactions (solved by Arrow IPC data_ref)
- dbt 1.9+ syntax deprecation (solved by pre-execution validation)
- Ungraceful PostgreSQL error handling (solved by SessionStart hook)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/data-platform/commands/explain.md

@@ -0,0 +1,44 @@
+# /explain - dbt Model Explanation
+
+Explain a dbt model's purpose, dependencies, and SQL logic.
+
+## Usage
+
+```
+/explain <model_name>
+```
+
+## Workflow
+
+1. **Get model info**:
+   - Use `dbt_lineage` to get model metadata
+   - Extract description, tags, materialization
+
+2. **Analyze dependencies**:
+   - Show upstream models (what this depends on)
+   - Show downstream models (what depends on this)
+   - Visualize as dependency tree
+
+3. **Compile SQL**:
+   - Use `dbt_compile` to get rendered SQL
+   - Explain key transformations
+
+4. **Report**:
+   - Model purpose (from description)
+   - Materialization strategy
+   - Dependency graph
+   - Key SQL logic explained
+
+## Examples
+
+```
+/explain dim_customers
+/explain fct_orders
+```
+
+## Available Tools
+
+Use these MCP tools:
+- `dbt_lineage` - Get model dependencies
+- `dbt_compile` - Get compiled SQL
+- `dbt_ls` - List related resources