fix(subagent): grow child-session transcript as one message instead of fragments

[justin-carper]

Problem

The TUI subagent pane fragments a subagent's flowing answer into many small messages. Live activity snapshots were posted as a new noReply user message on every flush — the 1.5s timer, every tool result, plus up to four more on finalize — so one subagent turn rendered as 5–20 fragments, e.g. a paragraph split mid-sentence:

A critical finding concerns
the IsStaged() method.
The IsStaged() change is mandatory…

Fix

The seeded prompt message's text part now grows in place: each flush PATCHes it via part.update (the same endpoint the child session's tool parts already use; opencode patches text parts in place and publishes part.updated, so live views re-render). Each flush carries the FULL cumulative transcript (replace, not append).

Also:

• Tool activity no longer duplicates into the transcript markdown — the child session's tool parts already render it live on the subagent card
• tool-result no longer forces a flush (was cutting paragraphs at every tool boundary)
• finalize merges resultSuffix + conversationSteps + activity line into the single cumulative transcript instead of three extra messages
• Falls back to the old per-flush new-message post only when the seed response has no parts or the PATCH fails (identical fallback posts deduped)

Result: child session renders as prompt + one live-updating message.

Carries the 0.9.1-next.0 pre-release bump (publishes to the next dist-tag; latest stays on 0.9.0). Revert-friendly if pre-release testing doesn't pass.

Verification: npm run typecheck clean, vitest run 621/621 pass (41 files), tsup build success.