github: credit all run actors as Co-Authored-By on commits

[dcramer]

Policy

When Junior makes a commit, the Co-Authored-By trailers must credit all actors in the run (run.actors), not just the run actor. If Alice asks for a change and Bob steers it mid-run, the commit was co-authored by both.

Implementation sketch

• Consume run.actors (specs/multi-actor-runs.md) wherever the github plugin / sandbox commit path composes commit messages. One trailer per actor, plus the existing agent-attribution trailer (repo convention in CLAUDE.md).
• Attribution only: run.actors must not influence which credentials push/commit (attribution-not-authority contract).

Open questions

• Trailer identity: Co-Authored-By: Name <email> needs an email per actor. Slack actors have optional contact/display fields; need a fallback (e.g. GitHub noreply derived from a linked identity, or omit actors with no resolvable email vs. use a placeholder). Decide and spec before implementing.
• Where commits actually happen today (sandbox git flows vs github plugin) — inventory the commit-composition sites first.
• Dedup rule when several actors resolve to the same email/identity.

Depends on

• Audit actor/instruction provenance across plugins #773 Track 1 (run.actors) — implemented on eval/multi-actor-provenance, pending merge.

Part of the #773 follow-up set: case-specific consumers of run.actors.