Apply workspace defaults to new blocks

[lyx-tec]

Summary

• Apply workspace default connection/cwd to new terminal and file browser blocks
• Add cwd fallback behavior for invalid or non-directory paths
• Improve cross-shell cwd initialization for POSIX, fish, and PowerShell

Tests

• go test ./pkg/shellexec ./pkg/jobmanager ./pkg/blockcontroller
• go test ./cmd/test-conn
• task package