[alycda]

Summary

This implements the proposal in #69. A quoted argument starting with @ stays a single board name, because it’s the only way to create a board containing spaces from the CLI, and it’s the PowerShell workaround from klaudiosinani/taskbook#41. The fix closes the gaps around that rule:

• Blank descriptions are rejected. get_options trims the parsed description and checks it once, replacing the two duplicate checks in create_task / create_note. tb -t " " and tb -n " " are rejected, and padded descriptions are stored trimmed.
• edit_description is trimmed too, so tb --edit @1 " " no longer blanks a description. This wasn’t in the issue; it’s the same hole.
• TaskbookError::MissingDescription replaces InvalidId(0), and main no longer prints it a second time, so users stop seeing Invalid item ID: 0 after the missing-description message.
• The quoting rules are documented in docs/cli-reference.md.

Testing

• cargo fmt --check, cargo clippy --all-targets (no warnings) and cargo test across the workspace all pass.
• Seven of the eight new Taskbook tests fail when the fix is reverted. The eighth, and two parser tests, pin the quoting rules on purpose: "@Work Projects" stays one board, and "Reply to @alice about the +1 p:3" stays literal.
• I ran the input matrix from Quoted @board arguments: board name absorbs the description, and whitespace-only tasks are stored #69 by hand against tb --cli.