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/ingest.md

@@ -0,0 +1,44 @@
+# /ingest - Data Ingestion
+
+Load data from files or database into the data platform.
+
+## Usage
+
+```
+/ingest [source]
+```
+
+## Workflow
+
+1. **Identify data source**:
+   - If source is a file path, determine format (CSV, Parquet, JSON)
+   - If source is "db" or a table name, query PostgreSQL
+
+2. **Load data**:
+   - For files: Use `read_csv`, `read_parquet`, or `read_json`
+   - For database: Use `pg_query` with appropriate SELECT
+
+3. **Validate**:
+   - Check row count against limits
+   - If exceeds 100k rows, suggest chunking or filtering
+
+4. **Report**:
+   - Show data_ref, row count, columns, and memory usage
+   - Preview first few rows
+
+## Examples
+
+```
+/ingest data/sales.csv
+/ingest data/customers.parquet
+/ingest "SELECT * FROM orders WHERE created_at > '2024-01-01'"
+```
+
+## Available Tools
+
+Use these MCP tools:
+- `read_csv` - Load CSV files
+- `read_parquet` - Load Parquet files
+- `read_json` - Load JSON/JSONL files
+- `pg_query` - Query PostgreSQL database
+- `list_data` - List loaded DataFrames