cli: rework +ssh-cache internals and user interface

This change primarily focused on a revised +ssh-cache user interface,
but it also reworks a bunch of the internals.

The primary CLI improvement is support for positional arguments and a
consistent list output format that includes both the ISO-formatted
timestamp and relative age.

    ghostty +ssh-cache                           # List all cached destinations
    ghostty +ssh-cache user@example.com          # Show that destination
    ghostty +ssh-cache example.com               # Show all users on that host
    ghostty +ssh-cache --add=user@example.com    # Manually add a destination
    ghostty +ssh-cache --remove=user@example.com # Remove a destination
    ghostty +ssh-cache --prune=30d               # Remove entries older than 30 days
    ghostty +ssh-cache --clear                   # Clear entire cache

Notable, we now support a --prune operation that replaces the previous
--expire-days flag that was never actually hooked up to anything (!!).
--prune also supports a wider range of Duration-based values.

We're also much more consistent with error codes: 0=success, 1=failure,
2=usage.

While working on those changes, I also reworked the cache internals,
particularly the code around timestamp handling and errors. For example,
I dropped the explicit error sets because they were growing unwieldy,
and in practice we only matched on a subset of those errors.

Lastly, overall test coverage should be much improved, especially around
the time- and allocation-related operations.

src/cli/ssh.zig

@@ -302,7 +302,7 @@ fn runInner(
 
     // Attempt to cache (if needed) on a successful ssh execution.
     if (exit_code == 0) if (session.to_cache) |entry| {
-        if (entry.cache.add(alloc, entry.dest)) |_| {
+        if (entry.cache.add(alloc, entry.dest, std.time.timestamp())) |_| {
             verbosePrint(opts, stderr, "cache: wrote {s}", .{entry.dest});
         } else |err| {
             log.debug("cache add failed for '{s}': {}", .{ entry.dest, err });