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

openspec/changes/ssh-key-mounting/.openspec.yaml

@@ -1,26 +1,27 @@
 name: ssh-key-mounting
-status: implementing
+status: implemented
 priority: high
 created_at: 2026-05-28
-updated_at: 2026-05-28
+updated_at: 2026-05-29
 labels:
   - feature
   - ssh
-  - config-profiles
+  - instances
 stories:
-  - title: Select SSH key in config profile
+  - title: Select SSH keys when creating/starting instances
     description: |
-      Add ssh_key_id to ConfigProfile so users can select an SSH key
-      to mount into container home directory (~/.ssh) when starting
-      a tool instance with that profile.
+      Add ssh_key_ids to ToolInstance so users can select multiple SSH keys
+      from a list when creating or starting a tool instance. Selected keys
+      are mounted into the container user's home directory (~/.ssh).
     acceptance_criteria:
-      - ConfigProfile model has nullable ssh_key_id column
-      - Config profile API accepts/returns ssh_key_id
-      - ResolvedProfile includes ssh_key_id
-      - start_instance mounts SSH key to {home_dir}/.ssh after applying profile
-      - Frontend config profile form has SSH key selector dropdown
-      - Git mount URL validation defaults to profile's SSH key
+      - ToolInstance model has nullable ssh_key_ids JSON column
+      - create_instance endpoint accepts ssh_key_ids list
+      - start_instance endpoint accepts ssh_key_ids override
+      - start_instance mounts all selected SSH keys to {home_dir}/.ssh
+      - Frontend CreateSessionForm shows multi-select SSH key checkboxes
+      - Frontend instance-list shows SSH key multi-select for start/restart
+      - SSH keys are validated (existence, user ownership) before mounting
     tests:
-      - unit: test_config_profile_resolver.py (resolver includes ssh_key_id)
-      - unit: test_tool_instances_legacy.py (ssh key mount integration)
+      - unit: test_tool_instances_legacy.py (existing baseline)
+      - integration: manual verification of mount behavior
     estimated_effort: small