Make Build a document view that drafts its own tasks

[MaggieAppleton]

What

Build becomes a calm third view (Document · Decisions · Build) in place of the old implementation panel, and asking the Planner for tasks no longer posts a message in Chat.

• One status line, then a checklist. Drafting → blocked (links to the decisions or comments to resolve) → review (Build on my laptop) → building (“started by @login”) → stopped (Try again / Return to planning) → “Built · N pull requests”. Rows show a state dot, the title and a #123 PR capsule, and expand to the goal and acceptance criteria.
• Tasks are drafted automatically, once each time someone opens Build. If the document changes while Build is open, it offers Update tasks instead of redrafting.
• No bookkeeping in the UI: no Refresh, no Prepare tasks, no workspace picker, no session IDs or raw state names. The viewer’s own available workspace is picked automatically (the server already filters to it).
• implementation:draft (new wire request, declared in packages/protocol/implementation.d.ts). The server writes the Planner instruction itself, checks write access the same way chat:send does, and keeps one live draft per document under the same send and queue caps. Chat shows only a system note such as “@ana asked Chopin to break the plan into tasks.”
• The implementation snapshot gains startedBy (the GitHub login).

This builds on main’s 05555f7b (Build as a workspace panel) and keeps its WorkspaceDocumentView plumbing and the Chat-left layout.

Not in this PR

Progress count on the segment, sync status, the one-click Build plan button, the builder role, and Jev auto-spikes. The living-document work will build those on top of this.

Review notes

• Design contract: the workspace.tsx sourceHash in scripts/design-contract/exceptions/dynamic-web.json is renewed. The ChatToggle and chat-presence classes it covers are unchanged. Maggie approved this.
• Security: reviewed for authorization (shared plannerWritable()), a server-written instruction, persisting before broadcasting, and turn spam (dedupe plus caps).

Tests

• bun test: 4463 pass, 4 skipped.
• bun run ci and bun run types: pass.
• E2E: implementation-launcher, harness, responsive-workspace, experiments, conversation-plan-layout and shell: 59 passed, 1 skipped (the opt-in live test).

Screenshots

How drafting works

sequenceDiagram
    participant B as Browser (Build view)
    participant S as Server
    participant P as Planner
    participant C as Chat
    B->>B: Open Build (once per visit, snapshot fetched this visit, chat history loaded)
    B->>S: implementation:draft {requestId, planRevision}
    S->>S: plannerWritable() — same write/role checks as chat:send
    alt a draft is already live for this document
        S-->>B: existing draft (deduped)
    else
        S->>C: persist system notice "@ana asked Chopin to break the plan into tasks."
        S->>P: queue Planner turn with server-authored instruction (send + queue caps)
        S-->>B: {state: queued | running}
        P->>S: edit_implementation_graph (draft tasks)
        S-->>B: implementation:drafting ended (after turn persisted)
        B->>S: GET snapshot → Review: "N tasks · Build on my laptop"
    end

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