feat: add config profiles data model and migrations

- Add ConfigProfile model with user ownership, name, description
- Add ConfigInclude model for ordered profile self-references
- Add ConfigMount model for mount/file definitions
- Add selected_profile_id to ToolInstance for per-instance profile selection
- Add default profile properties to UserConfig JSONB config
- Create Alembic migration 0013 for new tables and columns
- Register new models in models/__init__.py
- Mark config_folders.is_active as deprecated
- Add migration metadata test

Quality gates: syntax check passed (all files parse successfully)
OpenSpec: add-config-profiles task 1.1

openspec/changes/add-config-profiles/specs/tool-instance-profile-selection/spec.md

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+## ADDED Requirements
+
+### Requirement: Tool instance can have selected profile
+The system SHALL allow setting an explicit config profile on a tool instance.
+
+#### Scenario: Assign profile to instance
+- **WHEN** user sets selected_profile_id on a tool instance
+- **THEN** the instance stores the profile ID and uses it at launch time
+
+### Requirement: Tool instance uses default profile when none selected
+The system SHALL resolve a default profile for a tool instance when no explicit profile is selected.
+
+#### Scenario: Fallback to user default
+- **WHEN** a tool instance has no selected_profile_id
+- **THEN** the system uses the user's default profile for that tool type, or the global default
+
+### Requirement: Remove legacy auto-mount behavior
+The system SHALL no longer auto-mount the active config folder at tool launch time.
+
+#### Scenario: Launch without active folder
+- **WHEN** a tool instance launches with no selected profile and no default
+- **THEN** the instance starts without mounting any config folder