Create PR now understands multiple GitHub accounts

The Create Pull Request flow was rebuilt around account-aware GitHub routing. Kaji can parse GitHub remotes, discover the right account context, and show the selected account directly inside the PR modal instead of assuming every repo belongs to the same identity.

• Added GitHubAccount, GitHubAccountParser, GitHubAccountService, and GitHubRemoteURLParser
• Added CreatePRGitHubAccountField so the PR modal shows the account being used
• Refactored CreatePRModal into context, presenter, and chrome components
• Added PullRequestBranchNameSlug for cleaner generated branch names
• Added tests for GitHub account parsing and PR palette behavior

Agent instructions load without blocking the UI

Agent instruction discovery is now asynchronous, error-aware, and stricter about which directories it scans. Opening the instructions panel no longer has to stall while the app walks the workspace.

• Added AgentInstructionDocument, AgentInstructionDiscovery, and AgentInstructionPanelState
• Added AgentInstructionsPanel loading and error states
• Added directory filtering so generated and dependency folders are ignored
• Added Ask palette entries for opening agent instructions
• Added AgentInstructionDiscoveryTests

Shared modal components got cleaner

The command modal and form layout pieces were extracted into reusable components, which keeps future modal flows from piling more state and layout code into the main window.

• Added KajiCommandModalShell and KajiFormSection
• Moved modal coordination through KajiModalCoordinator and KajiModalRoute
• Simplified MainWindow modal presentation paths
• Bumped Kaji to 0.3.3