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/schema.md

@@ -0,0 +1,48 @@
+# /schema - Schema Exploration
+
+Display schema information for database tables or DataFrames.
+
+## Usage
+
+```
+/schema [table_name | data_ref]
+```
+
+## Workflow
+
+1. **Determine target**:
+   - If argument is a loaded data_ref, show DataFrame schema
+   - If argument is a table name, query database schema
+   - If no argument, list all available tables and DataFrames
+
+2. **For DataFrames**:
+   - Use `describe` to get column info
+   - Show dtypes, null counts, sample values
+
+3. **For database tables**:
+   - Use `pg_columns` for column details
+   - Use `st_tables` to check for PostGIS columns
+   - Show constraints and indexes if available
+
+4. **Report**:
+   - Column name, type, nullable, default
+   - For PostGIS: geometry type, SRID
+   - For DataFrames: pandas dtype, null percentage
+
+## Examples
+
+```
+/schema                    # List all tables and DataFrames
+/schema customers          # Show table schema
+/schema sales_data         # Show DataFrame schema
+```
+
+## Available Tools
+
+Use these MCP tools:
+- `pg_tables` - List database tables
+- `pg_columns` - Get column info
+- `pg_schemas` - List schemas
+- `st_tables` - List PostGIS tables
+- `describe` - Get DataFrame info
+- `list_data` - List DataFrames