feat(cli): generate man pages and shell completions (#7071)

Add the man-and-completions example, which writes photocraft-cli's
completions for bash, zsh, fish, elvish and PowerShell (clap_complete)
and its man pages (clap_mangen: photocraft-cli.1 and one per
subcommand, such as photocraft-cli-convert.1) into a directory:

  cargo run -p photocraft-cli --example man-and-completions -- <out-dir>

clap_complete and clap_mangen are dev-dependencies, so the binary is
unchanged. clap_mangen gets its env feature, so the pages name each
flag's environment variable as --help does; clap already has env, so
Cargo.lock gains no package for it. The library exposes only
`command()`, the clap definition; the args module stays private.

A test runs Command::debug_assert and checks that every shell's
completions and every page generate. Another repeats the example's two
generate_to calls and pins the exact file list: the five completion
files under the names the packages install, photocraft-cli.1, one page
per subcommand, and no pages for the help subcommand.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.toml

@@ -45,6 +45,8 @@ thiserror = "2"
 serde = { version = "1", features = ["derive", "rc"] }
 serde_json = "1"
 clap = { version = "4", features = ["derive", "env"] }
+clap_complete = "4.6.11"
+clap_mangen = { version = "0.3.3", features = ["env"] }
 arc-swap = "1"
 bytemuck = { version = "1", features = ["derive"] }
 flate2 = { version = "1", default-features = false, features = ["rust_backend"] }