Split overloaded CoS task metadata.context into prompt vs note fields

[atomantic]

metadata.context on a CoS task is overloaded — the same key carries two different kinds of content:

1. A one-line human note (server/routes/cosJobRoutes.js ~line 182; server/services/cosTaskStore.js ~line 920 escalation context, and the direct-write path at ~line 320: if (taskData.context) metadata.context = taskData.context;).
2. A multi-thousand-character agent prompt (server/services/cosTaskGenerator.js ~line 1609-1628: "Move the generator's multi-line prompt into metadata.context..." — confirmed still present: task.metadata.context = task.description;; consumed via server/routes/cosTaskRoutes.js ~line 197).

The prompt ends up there because generateTasksMarkdown flattens description to one line and parseTasksMarkdown only matches a - [ ] block's first line, so newlines in description would corrupt COS-TASKS.md on the markdown round-trip — metadata.context was the escape hatch that survives serialization.

Fix shape

Split into two fields:

• metadata.prompt — the full agent-facing payload (what cosTaskGenerator.js currently stuffs into metadata.context).
• metadata.context — reverts to just the one-line human note.

This touches:

• The markdown round-trip (generateTasksMarkdown/parseTasksMarkdown in server/services/cosTaskGenerator.js).
• LEGACY_DIRECT_FIELDS in server/services/cosTaskStore.js (~line 66, currently ['context', 'model', 'provider', 'effort', 'app']).
• server/services/cosTaskMerge.js (~line 200 area) — the merge logic that reconciles metadata on update.
• The agent prompt builder that currently reads the prompt out of metadata.context.
• Every on-disk task with an existing metadata.context prompt payload — needs a migration (scripts/migrations/) to move it to metadata.prompt so old tasks keep resolving correctly, per the root CLAUDE.md migration-required rule for on-disk format changes.

Deferred as disproportionate to what was a presentational problem (the long-text clamping in client/src/components/ui/CollapsibleText.jsx) — the UI clamp handles both shapes correctly today regardless of which one a given task uses, so there is no user-visible bug, only a data-modeling debt.

Migrated from PLAN.md by /do:replan --issues.

[atomantic]

Shipped in #4257.

Two decisions worth recording, both made autonomously:

One classification rule instead of per-producer edits. The issue names cosTaskGenerator and the cosTaskRoutes claim/JIRA prompts, but the same overloading exists in referenceRepos, repoIntake, autoFixer, agentWorktreeCleanup and the challenge-escalation filer — all of which pass a multi-line body as context. Rather than rewrite every producer (and every test asserting on their call shape), the rule lives once at the store boundary: isPromptPayload in the new server/lib/cosTaskPrompt.js says a value with a newline is a prompt, because the context contract is explicitly one line. cosTaskStore.addTask applies it on both the direct-write and raw paths, and the migration calls the same predicate, so a task cannot be classified one way at creation and the other way on disk. Producers keep their existing call shape; the two the issue names now pass prompt explicitly.

Classification is create-only. updateTask leaves both fields alone, because the task editor's textarea is seeded from the note — re-classifying a multi-line edit of it would silently overwrite the task's real prompt.

No prompt-template migration. cos-agent-briefing.md addresses {{task.metadata.context}}, and so does every copy an install has customized. Instead of pushing a template change plus a PROMPT_VERSIONS-style migration onto every install, buildAgentPrompt folds prompt + note back into that key at render time; metadata.prompt still travels for a custom template that wants it. Related: cos-evaluate.md (the triage prompt) now sees the one-line note instead of an inlined multi-thousand-character body — a strict improvement for a selection prompt, and the reason it needs no migration either.

Compatibility: reads go through getTaskPrompt, which prefers metadata.prompt and falls back to metadata.context, so pre-split tasks and peers on older code resolve unchanged. scripts/migrations/270-cos-task-prompt-split.js rewrites existing queues, and PORTOS_SCHEMA_VERSIONS.cosTasks goes 4 → 5 so an unmigrated peer skips cos-task sync rather than mis-running a task whose body it cannot read.