Add agent selection and security hardening

[dalferr]

Summary

• Add AI agent selection when creating a workspace.
• Persist the selected agent_command and auto-approve option.
• Ensure workspace names remain unique per project and agent.
• Harden project initialization, imports, paths, sessions, and tmux handling.
• Add TUI flow, integration, regression, and harness isolation tests.
• Bump the application version to 0.9.1.

Validation

• go test ./...
• go test -race ./internal/project ./internal/initcmd ./internal/tui
• go vet ./...
• git diff --check
• make lint
• make build and ./bin/crabby --version

The test harness isolates HOME and PATH; the real Crabby registry remained unchanged after the test suite.

Excluded

• scripts/e2e/
• .agents/
• Planning and infrastructure documents
• Local build artifacts