Consolidate bundled plugin schema into the core migration chain

The core chain already owns and reproduces the full bundled schema (deploys run
`flask db upgrade` only). The per-plugin Alembic baselines duplicated those
tables, so `flask plugin upgrade-all` would conflict - a footgun. Remove the 6
bundled plugin migration dirs; the per-plugin Alembic helpers
(alembic_template, PluginMigrationRunner) remain for external/filesystem
plugins. upgrade-all now cleanly no-ops for bundled plugins.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plugins/network/migrations/versions/0001_baseline.py

@@ -1,27 +0,0 @@
-"""network plugin: baseline schema
-
-Creates every table owned by the network plugin per
-shopdb.plugins.alembic_template.PLUGIN_TABLE_OWNERS. The table definitions
-are derived from the SQLAlchemy models at migration runtime so this stays
-in lockstep with the model layer without duplication.
-
-Revision ID: 0001_baseline_network
-Revises:
-Create Date: 2026-05-30
-
-"""
-from shopdb.plugins.alembic_template import create_plugin_tables, drop_plugin_tables
-
-
-revision = '0001_baseline_network'
-down_revision = None
-branch_labels = None
-depends_on = None
-
-
-def upgrade():
-    create_plugin_tables('network')
-
-
-def downgrade():
-    drop_plugin_tables('network')