fix: support steering in lean TUI

[rumpl]

Summary

• route busy lean TUI editor submissions through the runtime steering queue
• render pending steering/follow-up messages at the end of the live stream with muted styling
• reconcile runtime user-message confirmations so pending steering messages do not duplicate

Tests

• go test ./pkg/app ./pkg/leantui ./pkg/tui/page/chat
• task lint
• task --force build
• task test (fails in pkg/model/provider: TestOpenAIAliasProvider_LiveAPI/moonshot returns HTTP 404 / permission denied for kimi-k2-0905-preview)

[docker-agent]

Assessment: 🟢 APPROVE

The steering queue routing, pending-message reconciliation, and deduplication logic introduced in this PR are all correct.

Key paths reviewed:

• dispatchUserMessage correctly routes busySubmitSteer via app.Steer and records a pending entry with display/content fields.
• handleUserMessageEvent correctly consumes the pending entry by matching on content (not display), so the user-facing display text (e.g. /change) is preserved when echoing while the resolved content is used for matching.
• samePendingUserContent correctly handles the trailing \n that the runtime appends to emitted messages via strings.TrimSuffix, so reconciliation succeeds reliably.
• finishBusy correctly dequeues follow-up messages and promotes their pending display text before echoing.
• Error from app.Steer is surfaced to the user via a notice; no silent failure.

No bugs found in the changed code.