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/.mcp.json

@@ -0,0 +1,10 @@
+{
+  "mcpServers": {
+    "data-platform": {
+      "type": "stdio",
+      "command": "${CLAUDE_PLUGIN_ROOT}/mcp-servers/data-platform/.venv/bin/python",
+      "args": ["-m", "mcp_server.server"],
+      "cwd": "${CLAUDE_PLUGIN_ROOT}/mcp-servers/data-platform"
+    }
+  }
+}