feat: instance-level SSH key selection for container mounting

- Revert mistaken ssh_key_id from ConfigProfile (model, API, resolver, frontend)
- Add ssh_key_ids JSON column to tool_instances via migration
- Update create_instance to accept and store ssh_key_ids
- Update start_instance to mount selected SSH keys to {home_dir}/.ssh
- Update list_instances to return ssh_key_ids
- Frontend CreateSessionForm: multi-select SSH key checkboxes
- Frontend instance-list: SSH key selector for start/restart actions
- Maintain separate SSH key dirs per key to avoid conflicts

Quality gates: pytest (231 passed, 6 pre-existing), tsc --noEmit clean

apps/api/alembic/versions/2026_05_29_add_ssh_key_ids_to_tool_instances.py

@@ -0,0 +1,27 @@
+"""add_ssh_key_ids_to_tool_instances
+
+Revision ID: 2026_05_29_add_ssh_key_ids_to_tool_instances
+Revises: 2026_05_29_drop_ssh_key_id_from_config_profiles
+Create Date: 2026-05-29 12:46:00.000000
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = "2026_05_29_add_ssh_key_ids_to_tool_instances"
+down_revision = "2026_05_29_drop_ssh_key_id_from_config_profiles"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "tool_instances",
+        sa.Column("ssh_key_ids", sa.JSON(), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("tool_instances", "ssh_key_ids")

apps/api/alembic/versions/2026_05_29_drop_ssh_key_id_from_config_profiles.py

@@ -0,0 +1,32 @@
+"""drop_ssh_key_id_from_config_profiles
+
+Revision ID: 2026_05_29_drop_ssh_key_id_from_config_profiles
+Revises: 069d3da4dc9b
+Create Date: 2026-05-29 12:45:00.000000
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = "2026_05_29_drop_ssh_key_id_from_config_profiles"
+down_revision = "069d3da4dc9b"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.drop_column("config_profiles", "ssh_key_id")
+
+
+def downgrade() -> None:
+    op.add_column(
+        "config_profiles",
+        sa.Column(
+            "ssh_key_id",
+            sa.Uuid(),
+            sa.ForeignKey("ssh_keys.id", ondelete="SET NULL"),
+            nullable=True,
+        ),
+    )