docs(ledger): document the 8-state Task workflow as it actually runs in prod

## Context
The state machine grew organically. Prod distribution across 314 tasks: BACKLOG, READY_FOR_PLANNING, PLANNING_VALIDATED, IN_PROGRESS, IN_REVIEW, AWAITING_HUMAN, DONE, CANCELED. Yesterday's plan referenced a stale "5-state" model. New team members and the dispatcher itself would benefit from a canonical doc.

## Acceptance Criteria
- [ ] Doc at `operant-studio/docs/task-state-machine.md`
- [ ] Diagram of legal transitions (draw with mermaid)
- [ ] For each state: entry criteria, exit criteria, who/what triggers each transition (human vs. dispatcher vs. planner vs. reviewer)
- [ ] Notes on `AWAITING_HUMAN` semantics (what unblocks it)
- [ ] Notes on `CANCELED` vs `DONE` vs soft-delete
- [ ] Link from README

## Files Touched
- operant-studio/docs/task-state-machine.md (new)
- operant-studio/README.md (add link)

## Dependencies
- None

## Verification
Peer read by Sweetness (or reviewer of choice).