fix(tui): render instruction updates as compact notices - #41900

What

Instruction-update notices in the TUI now always render as a compact one-liner (◈ Instructions updated: core/codemode) instead of sometimes dumping the entire model-facing update text — up to a full multi-hundred-line Code Mode catalog — into the transcript.

Before / After

Before: the TUI had two divergent paths for session.instructions.updated, and which one you saw depended on how the message reached you:

• Live: while connected, the client-side event fold synthesized a system message with compact text Instructions updated: <source keys>. This is the small notice users are used to.
• Projected: on session load or reconnect — which is always the path after a server restart — the projected message carries the full frozen model-facing text, and SessionNoticeMessageV2 rendered message.text verbatim. A restart-triggered Code Mode catalog update therefore appeared as a wall of text.

The verbatim render regressed in #36254 (July 10), which replaced the previous hardcoded "Instructions updated" label with props.message.text. The two paths also disagreed on whether to show a message: the projector skips text-less deltas (initial baseline, empty renders), but the live fold only skipped the initial baseline — so a live-shown notice could silently vanish on reload.

After: both paths produce the same message shape and the renderer never shows the full text:

• The projected System message gains an optional description — Instructions updated: <delta keys> — built by the projector from the durable event's delta.
• The live fold emits the same shape (full text, compact description) and now skips text-less deltas exactly like the projector.
• SessionNoticeMessageV2 renders description ?? "Instructions updated" for system messages. The plain-label fallback covers messages projected before this change.

The model-facing contract is untouched: message.text still carries the full frozen update, which to-llm-message.ts sends as the system message on every request rebuild and compaction summarizes. Only human display changes.

How

• packages/schema/src/session-message.ts — Session.Message.System gains optional description, mirroring Synthetic.
• packages/core/src/session/message-updater.ts — projector populates description from event.data.delta keys.
• packages/tui/src/context/data.tsx — live event fold mirrors the projector: skips text: undefined events, stores full text plus compact description.
• packages/tui/src/routes/session/index.tsx — SessionNoticeMessageV2 shows description ?? "Instructions updated", never text.
• packages/client/src/promise/generated/types.ts — regenerated (bun run generate from packages/client).

sequenceDiagram
    participant Core as Projector (core)
    participant TUI as TUI event fold
    participant View as SessionNoticeMessageV2

    Note over Core,TUI: session.instructions.updated { delta, text? }
    Core->>Core: skip if text undefined
    Core->>View: System { text, description: "Instructions updated: keys" }
    TUI->>TUI: skip if text undefined (now matching)
    TUI->>View: same shape, synthesized live
    View->>View: render description ?? "Instructions updated"

Scope

• Does not change what the model sees: the frozen full text remains the durable payload and the LLM-request content.
• Does not backfill description onto previously projected messages; they fall back to the plain "Instructions updated" label. Projections are rebuildable if exact parity is ever wanted.
• The upstream cause of the most common phantom notice (restart snapshot race) is fixed separately in fix(core): gate tool snapshot on initial MCP registration #41884; this PR makes even legitimate updates render compactly.

Testing

From package directories:

• packages/schema, packages/core, packages/tui, packages/client: bun typecheck — clean.
• packages/core: bun test test/instruction-state.test.ts test/session-runner.test.ts test/bus.test.ts — 204 pass.
• packages/tui: bun test test/cli/tui/data.test.tsx test/cli/tui/inline-tool-wrap-snapshot.test.tsx — 52 pass. The data test previously asserted the divergent live behavior (message for a text-less delta, compact string stored in text); it now asserts the unified shape and the text-less skip.