board: 19 → in-progress (istos) — complete the half-committed move

The claim commit e31de61 was hand-made after the board (gate still off)
had already moved the file on disk: it added the in-progress copy but
missed staging the backlog deletion. Nothing was lost — same content,
both paths, now one.

tasks/backlog/19-boards-sync-through-origin-main.md

@@ -1,99 +0,0 @@
-# 19 — Boards sync through origin/main: push on move, pull on a beat
-
-**Status:** Backlog
-**Priority:** High — this is the multi-user feature; 18 without it is bookkeeping
-**Type:** Feature
-**Depends on:** 18 — the claim commit is what gets pushed, and its
-same-commit atomicity is what makes the races below resolve correctly
-
-One machine's `tasks/` is the truth today; everyone else's is stale
-until someone remembers to push and pull. Make origin/main the truth
-and every board a converging replica: board-made moves push
-immediately, every board pulls on a short beat, remote moves appear in
-the ticker attributed to their author, and losing a same-card race is
-a toast, not a mystery.
-
-## Context
-
-- `watch.py` polls the stage directories every 2s — remote changes
-  that arrive via pull are already noticed and narrated; today they
-  would read "disk", this card upgrades them to the commit author.
-- Task 14 (landed) already points fresh agent branches at
-  origin/main; this card gives the *board state* the same treatment.
-- The precondition that makes pulling safe is bench's own discipline:
-  code work lives in worktrees and PRs, so the main checkout stays
-  clean and fast-forwardable. Team mode assumes — and the docs must
-  say — that local main advances only through the board and origin.
-- Push publishes every local-ahead commit, not just the board's —
-  the piggyback hazard below is the sharpest edge in this card.
-
-**Affected areas:** a new small `sync.py` (or a sibling thread beside
-`watch.py`), `config.py` (settings), `state.py`/ticker attribution,
-AGENTS.md (team-mode discipline).
-
-## What to build
-
-- **Gate**: `BOARD_SYNC=1` (default off; implies `BOARD_COMMIT_MOVES`).
-  Off = today's behaviour exactly.
-- **Push, event-driven**: after each board-made task commit, push. On
-  non-fast-forward: fetch, rebase the board commits, push again. If
-  the rebase conflicts on a task file, the local move loses: revert
-  it, re-read the remote version, and toast who took it
-  ("07 claimed by elena — your move was undone").
-- **Piggyback guard**: before any auto-push, every local-ahead commit
-  on main must be `board: `-prefixed. Anything else → no push, one
-  clear ticker warning naming the stray commit. Never publish a
-  human's unpushed work as a side effect.
-- **Pull, periodic**: fetch + fast-forward-only merge every
-  `BOARD_SYNC_INTERVAL` (default ~30s) and once at startup. Never
-  pull into a non-clean tree or past a divergence — narrate and wait
-  for a human instead. The watcher then narrates arrived moves with
-  the commit author's name instead of "disk".
-- **Offline**: unreachable origin degrades to local-only silently
-  sane — commits queue, a quiet ticker note says sync is behind,
-  next successful fetch catches up. No errors every 30s.
-
-**Out of scope** — tempting neighbours left alone:
-
-- Reacting to synced state with side effects — replicas render only
-  (task 20 owns the etiquette).
-- Syncing `local/state/` — liveness stays per-board by design.
-- Any transport other than git via origin; any branch other than main.
-
-## Acceptance
-
-- [ ] Given two clones with `BOARD_SYNC=1`, when A moves a card, then
-      within one beat B's board shows the move and B's ticker
-      attributes it to A's git name.
-- [ ] Given both move the same card in one window, then exactly one
-      claim survives on origin, the loser's board reverts with the
-      take-over toast, and both converge to the same file bytes.
-- [ ] Given a human's non-board commit sitting on local main, when
-      the board would push, then it refuses with the named warning
-      and the human's commit stays private.
-- [ ] Edge case — origin unreachable: boards keep working locally,
-      one quiet ticker note, full convergence after connectivity
-      returns; no error spam.
-- [ ] With the gate off, no fetch, no push, no behaviour change.
-
-## Open questions
-
-- None.
-
-## Notes
-
-Middle card of the multi-user arc (18 claim → 19 sync → 20 etiquette).
-Git is the lock server and main the linearizer: push races are the
-concurrency control, which is why the claim must ride in the move's
-own commit (18) and why replicas must not react (20).
-
-**Risks**
-
-- ff-only pulls plus the piggyback guard mean a divergent main stalls
-  sync until a human resolves it — correct, but the stall must be
-  loudly visible or two halves of a team quietly stop converging.
-- A 30s beat × N boards is N fetches/min against origin — fine for
-  GitHub, worth a doc line for rate-limited setups.
-- Reverting a lost move while the user is mid-drag needs care in the
-  UI — the board re-reads disk on every render, which should make
-  this safe, but test the race deliberately.