[penberg]

Unifies what the three tools print: errors, warnings, status lines and usage. Each had grown its own shape; now every message is prog: subject: problem, on one line, in lowercase, so that people and agents can read it, and it is still as short as a UNIX tool's.

Case	Before	After
name taken	fsfork: /private/…/.p.x.base: File exists (os error 17)	fsfork: p.x: file exists
base left over	the same	fsfork: p.x: its base .p.x.base is left over; remove it
missing	fsfork: nope: No such file or directory (os error 2)	fsfork: nope: no such file or directory
not a directory	fsfork: /private/…/p/a: not a directory	fsfork: p/a: not a directory
not a fork	fsdiff: /abs/p: no base at /abs/.p.base; is it a fork?	fsdiff: .: not a fork: no .p.base next to it
bad option	Unrecognized argument: --bogus, a blank line, Run fsfork --help for more information.	fsfork: unrecognized argument: --bogus and the usage line
conflicting options	confine: --rm removes a fork, and -d runs the command without one	confine: --rm cannot be given with -d, which makes no fork and the usage line
no such command (macOS)	sandbox-exec: execvp() of 'nosuchcmd' failed: …, exit 71	confine: nosuchcmd: command not found, exit 127
no Landlock (Linux)	confine: confine needs Landlock, …	confine: Landlock is not available; it needs Linux 5.19 or later

• Capitalization. The system's error text is lowercased and loses Rust's (os error N), so that it matches our own text.
• Paths. A path is shown as it was typed. A path the command made itself, such as a fork's, is shown as it would be typed from the working directory: p.x, or ../turso.1. forked to still gives the full path, because the fork is named only there, and the command then runs somewhere else.
• Usage errors print the man page's synopsis after the problem: the single most useful line for fixing the arguments.
• confine looks for the command first. A command that isn't there now exits 127, as the man page says, and is checked before forking, so it leaves no fork. A path with a / in it is looked for in the directory the command runs in.
• The git fallback warning is printed by the command that forked, so from confine it now says confine: and not fsfork:. It shows only the first line of git's error.

The message formatting lives in fsfork::message, which all three commands share. The lib now depends on argh, for parsing.

Stacked on #7, since it changes the same fsfork.rs. Merge that first.

Not changed

• --help output is still argh's (Usage:, Options:). It goes to stdout and isn't a message.
• On Linux, errors from cp during a fork are passed through as cp prints them.

Testing

cargo test and cargo clippy pass on macOS. New tests check exact stderr for usage errors in all three commands, for lowercase system errors with paths as typed, for taken names and left-over bases, and that confine says a missing command is not found, exits 127 and makes no fork. CI covers Linux. The Landlock message hasn't been seen on a machine without Landlock.