Add create_with_author to Ledger trait

[jdc-pub]

The Ledger::create method currently uses self.signature() (the repo's git config) for both author and committer with no way to override them. For import use cases — e.g. syncing issues from GitHub into a local forge store — it's necessary to preserve original authorship and timestamp on the resulting commit.

Proposed API

fn create_with_author(
    &self,
    ref_prefix: &str,
    strategy: &IdStrategy<'_>,
    fields: &[(&str, &[u8])],
    message: &str,
    author: &git2::Signature<'_>,
) -> Result<LedgerEntry, Error>;

The committer would remain self.signature() (the local identity); only the author would be overridden.

Motivation

When importing GitHub issues, authorship should be attributed to the original author:

• name: user.login
• email: <login>@users.noreply.github.com
• time: created_at from the GitHub payload

Without this, all imported commits appear authored by whoever ran the import.

Assisted-by: Claude Code (Claude Sonnet 4.6)