feat: add MarkFlagNonRepeatable to reject repeated flags

[aspr0d]

Summary

Adds MarkFlagNonRepeatable(name string) to Command, which prevents a flag from being specified more than once on the command line.

Currently, developers who need this behavior must define custom pflag.Value types for every flag type they use (~60 lines each), as described in #2355. This PR offers a single-call API that wraps the flag's existing Value to reject a second Set() invocation.

Implementation

• nonRepeatableValue — a thin wrapper around pflag.Value that counts Set() calls and returns an error on the second one.
• IsBoolFlag() is forwarded so --verbose (no value) still works for boolean flags.
• The approach catches the repetition at parse time, giving the user immediate feedback.

Example

cmd.Flags().String("name", "", "user name")
cmd.MarkFlagNonRepeatable("name")
// mycli --name=alice --name=bob → error

Test plan

• TestNonRepeatableFlag — 5 cases: single string, repeated string, single bool, repeated bool, no flags
• All existing tests pass (go test ./...)
• go build ./... and go vet ./... clean

Closes #2355