feat: implement repository clone mode with SSH key support

- Add clone_mode and branch fields to tool_instances
- Add ssh_key_id to git_repositories for per-repo SSH key assignment
- Implement host-side git cloning with branch selection (default: main)
- Mount SSH keys into containers for git operations in clone mode
- Add dirty state check on clone-mode instance deletion with confirmation
- Update SessionsPage with mount/clone selector, branch input, SSH key display
- Add SSH key selector to repository creation form
- Add dirty delete confirmation modal with changed files list
- Update API schemas and endpoints for new fields
- Sync delta specs to main specs (git-repo, tool-instances, repo-clone-mode)
- Archive completed OpenSpec change: repo-clone-mode-with-ssh
- Document git requirement for custom tool types

Quality gates: Frontend typecheck and build passed
OpenSpec: repo-clone-mode-with-ssh archived with all tasks complete

openspec/changes/sessions-hub/proposal.md

@@ -1,51 +0,0 @@
-# Sessions Hub
-
-## Problem
-
-Users currently have to navigate into individual projects and repositories to see their active tool instances (sessions). There's no centralized place to:
-- See all active/open sessions at a glance
-- Quickly access the last used session
-- Create new sessions without navigating deep into the project hierarchy
-
-## Solution
-
-Create a dedicated **Sessions Hub** page that serves as the central place for managing tool instances:
-
-1. **Navigation tab** between Dashboard and Projects
-2. **Active sessions section** showing all running/open instances
-3. **Last session** prominently displayed for quick access
-4. **Quick create** - create sessions for any project from one place
-5. **Persist last session** in user config for easier access
-
-## Key Features
-
-### Sessions Page
-- Shows all active (running) sessions with status, type, and links
-- Shows recent sessions (last 5)
-- Shows last created session at the top for quick access
-- "New Session" button to create instances for any project
-
-### Navigation
-- New "Sessions" tab in the app shell between Dashboard and Projects
-- Shows count of active sessions as a badge
-
-### Quick Access
-- Last created session saved to user config
-- One-click to reopen/resume last session
-- Session history for quick navigation
-
-## Benefits
-
-- **Faster workflow** - No need to navigate deep into projects
-- **Better visibility** - See all active work at a glance
-- **Quick resume** - Jump back to last work instantly
-- **Centralized management** - One place for all sessions
-
-## Success Criteria
-
-- [ ] Sessions tab visible in navigation between Dashboard and Projects
-- [ ] Sessions page shows active sessions
-- [ ] Last session displayed prominently
-- [ ] Can create session for any project from Sessions page
-- [ ] Last created session persists in user config
-- [ ] Badge shows count of active sessions