github: create resources as Junior, not as the user; prefer Junior's own identity for side effects

[dcramer]

Policy

GitHub resource creation (PRs, issues, comments, commits/pushes) should go back to acting as Junior (app/installation/service-principal identity), not as the requesting user via their OAuth token. More broadly, this is the default intent everywhere: Junior acts as itself where possible; delegated user credentials are the exception, reserved for operations that inherently require the user's identity (user-private resource access, user-consented reads).

Humans get credited through attribution metadata, not borrowed identity: Co-Authored-By trailers for all run actors (#777), "requested by" references in bodies/descriptions, and creator audit fields — the same split the scheduler already uses (system actor executes, createdBy is audit metadata; see specs/identity.md).

Rationale

This is the agent-as-teammate answer to the confused deputy problem (#773). Acting as the user makes every multi-actor run a delegation problem: which user's token, what happens when another actor steers, what the audit trail means. Acting as Junior makes execution authority uniform and auditable regardless of how many actors are in the run, and turns human attribution into explicit, verifiable metadata. It also shrinks the surface that point-of-action authority checks (#773 section D) must guard — operations under Junior's own identity have no cross-actor credential conflict by construction.

Scope

• Inventory which GitHub operations currently execute with user-delegated tokens vs app/installation identity; revert creates to Junior's identity.
• State the general default in specs/identity.md (and cross-reference from specs/credential-injection.md): service-principal/system identity is the preferred execution identity; delegated user credential subjects only where the operation requires user identity, per existing spec exceptions.
• Verify attribution lands: github: credit all run actors as Co-Authored-By on commits #777 trailers on commits; requester references on PRs/issues created as Junior.

Open questions

• Repo permissions: Junior's app/installation identity needs write access wherever users previously supplied it via their own tokens.
• Notification semantics change (created-by-Junior does not @-notify as the user would); decide whether bodies should mention the requesting actor for visibility.
• Any GitHub surfaces that genuinely require user identity (e.g. reviews that must come from a human) — enumerate and keep those on delegated credentials explicitly.

Related

• Audit actor/instruction provenance across plugins #773 (confused deputy / point-of-action authority), github: credit all run actors as Co-Authored-By on commits #777 (Co-Authored-By all run actors), Retire 'requester' vocabulary: complete the actor rename across code and specs #778 (system actors in actorSchema — gives run.actor a first-class value for Junior-as-itself runs)

Part of the #773 follow-up set.