fix: normalize verified trailers and preserve amend arguments

[chtnnh]

Problem

Commit helpers leave stale Know-Code-Verified trailers in place and can append more than one trailer for multiple message arguments. Amend reconstruction can discard valid non-message options such as author, date, or signing flags.

Scope

Centralize trailer replacement and preserve all supported amend arguments.

Acceptance criteria

• Exactly one current grounded trailer is present after commit/amend preparation.
• Stale and duplicate trailers are replaced deterministically.
• Multiple -m messages do not receive duplicate trailers.
• Amend keeps non-message flags and only replaces message-bearing arguments.
• Tests cover stale trailers, duplicate trailers, multiple messages, and amend options.