Hide the Alt+C command line by default

[jasonsuhari]

Problem

The dedicated command line always reserves a terminal row even when it is not focused. This reduces the grid area before the user opens the command line.

Requested behavior

• Start with the command line hidden.
• Do not reserve a row while it is closed.
• Keep Alt+C opening and closing the command line.
• Preserve command output expansion while the command line is open.

Acceptance checks

• The initial layout gives the command-line row back to the pane grid.
• Alt+C reveals a one-row command line.
• Pressing Alt+C again hides it.
• Formatting and relevant Rust validation pass.

[jasonsuhari]

PR #277 is merged and all local/CI validation passed. Reopening this issue only for the required local follow-up: the checked-out main worktree currently contains active, unrelated Ports/resume edits and is four commits behind origin/main, so it was not safe to fast-forward or run npm run install:local without disturbing that work.

The merged commit is already on origin/main; once the active dirty worktree is clear, fast-forward local main, validate there, run the local installer, and close this issue.