feat(nix): install man pages from cobra command tree

[cpcloud]

Install man pages as part of the Nix package build.

Context

After the recent cobra/styling changes, the CLI has well-structured command descriptions and flag help. Cobra supports generating man pages via cobra/doc, so we can now produce and install them.

Scope

• Generate man pages from cobra command tree (e.g., micasa.1, micasa-backup.1, micasa-config.1)
• Install them in the Nix package output ($out/share/man/man1/)
• NixOS module should add them to environment.systemPackages or equivalent so man micasa works
• Consider shell completions (bash, zsh, fish) in the same pass if not already handled