quest: plan spawn skipping finished line children

quest/m0/README.md

@@ -32,5 +32,6 @@ ownership-manifest design:
 
 - [Quest on Cloudflare](/quest/m0/cloudflare/README.md) - contributors lock quests and donate tokens to run them; maintainers review and merge in a web UI synced with GitHub
 - [Quest log theme](/quest/m0/theme/README.md) - an MMO-style identity, copy, board, landing page, and CLI output for the demo and launch
+- [Spawn skips finished line children](/quest/m0/spawn-line-children.md) - `/quest-spawn` offers no quest already finished on its line branch, and spawned agents keep scratch files apart
 - [Release proof](/quest/m0/release-proof.md) - a fresh repository completes the whole lifecycle in CI, then v0.1.0 is tagged
 - [Launch material](/quest/m0/launch.md) - README, quickstart, demo, comparison, and launch-post drafts

quest/m0/spawn-line-children.md

@@ -0,0 +1,24 @@
+# [S] Spawning skips finished line children and isolates agent scratch
+
+## Goal
+
+`/quest-spawn` never offers a quest that is already finished on its
+questline's open branch, and the agents it spawns never share scratch files.
+
+## Plan
+
+Seen in moq-dev/moq on 2026-10-06, spawning 32 quests at once:
+
+- `quest ready` reads only the base branch. A questline that lands children
+  into its own open branch (moq-dev/moq#4519, #4653) deletes each finished
+  child there, but the child stays ready on the base until the line merges.
+  Four of the 32 offered quests were already done, and one agent was spawned
+  on one before it noticed.
+- Spawned agents wrote logs and PR bodies to the parent session's scratchpad
+  under common names, so runs overwrote each other's `check.log` and two PRs
+  went up with each other's descriptions.
+
+Open for the implementer: whether `ready` itself consults remote line
+branches (it is offline and git-only today) or the spawn skill checks them,
+and how the spawn prompt keeps scratch files inside each agent's worktree.
+Prefer the shape that keeps the core independent of a forge.