feat: contributor assignment and revision loop

[chubes4]

Goal

Let an editor assign work to a contributor and carry it through draft, review, revision, and publication without coordinating status through email or personal memory.

Account and team access are explicitly granted by administrators through Extra Chill Users. That is outside this issue.

Current foundation

Studio already provides:

• born-on-main drafts and autosaves
• contributor-owned draft picker
• preview and Submit for Review
• editor-facing pending review queue
• native WordPress editing and publication
• publication notifications

Missing workflow

• editor creates an assignment with a brief and assignee
• contributor sees assigned work and its current status
• assignment links to one canonical main-site draft
• editor sees who is working on what
• editor requests revisions with a note
• contributor sees the request and resubmits
• both sides can see whether work is assigned, drafting, in review, awaiting revision, scheduled, or published

Investigation requirement

Before adding storage, inspect WordPress Core and Gutenberg primitives for posts, authors, pending status, revisions, autosaves, comments/discussion, post meta, and notifications. Inspect current Studio review and receipt-backed notification contracts. Build only the coordination gap; do not create a parallel editorial CMS or duplicate post lifecycle.

First vertical slice

1. Editor creates an assignment for an existing contributor.
2. Assignment creates or links the contributor's canonical main-site draft.
3. Contributor opens the assignment in Studio and writes in Compose.
4. Submit for Review updates the assignment state.
5. Editor requests revisions with a note or approves for normal publication.
6. Contributor resubmits after addressing the note.
7. Publication closes the assignment and links the live article.

Acceptance criteria

• Only authorized editors can assign work or request revisions.
• Contributors can only access assignments and drafts permitted by native capabilities.
• Every assignment links to exactly one canonical main-site post.
• Existing autosave, revision, review queue, and publication behavior remain authoritative.
• State transitions are idempotent and auditable.
• Revision notes notify the contributor through the shared notification system.
• Editors have a focused view of active assignments and blockers.
• Contributors have a focused view of their assignments, revision requests, and completed work.
• Tests cover assignment, unauthorized access, submit, revision request, resubmit, publication, and replayed transitions.

Out of scope

• granting team or site access
• user provisioning or role management
• portfolios, gamification, or performance scoring
• social publishing approvals
• real-time collaborative editing