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/computers/migrations/script.py.mako

@@ -1,23 +0,0 @@
-"""${message}
-
-Revision ID: ${up_revision}
-Revises: ${down_revision | comma,n}
-Create Date: ${create_date}
-
-"""
-from alembic import op
-import sqlalchemy as sa
-${imports if imports else ""}
-
-revision = ${repr(up_revision)}
-down_revision = ${repr(down_revision)}
-branch_labels = ${repr(branch_labels)}
-depends_on = ${repr(depends_on)}
-
-
-def upgrade():
-    ${upgrades if upgrades else "pass"}
-
-
-def downgrade():
-    ${downgrades if downgrades else "pass"}