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/src/models/tool_instance.py

@@ -59,6 +59,7 @@ class ToolInstance(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     selected_config_profile_id: Mapped[uuid.UUID | None] = mapped_column(
         UUID(), ForeignKey("config_profiles.id", ondelete="SET NULL"), nullable=True
     )
+    ssh_key_ids: Mapped[list[str] | None] = mapped_column(JSON, nullable=True)
 
     tool_type: Mapped["ToolType"] = relationship()
     repository: Mapped["GitRepository"] = relationship()