feat(agentic): route PlaintextFollowup Write through post-write Read

[bobleer]

Summary

• PlaintextFollowup Write no longer echoes generated file bodies in tool results or persisted assistant tool calls; the runtime injects a synthetic Read after each generated Write so the next model round continues from normal Read output.
• Fail fast before slow Write content generation when Read is unavailable or unregistered, and align agent prompts plus tool-loop/truncation guidance to prefer Read + Edit over repeated Write on the same path.
• Resolve auto/default model selectors to primary (remove short first-turn fast-model shortcut) and track Write stats via lines_written.

Test plan

• cargo test -p bitfun-core round_executor -- --nocapture
• Manual PlaintextFollowup flow: Write(file_path) → content generation → Write execution → auto Read → Edit from Read result
• Confirm ACP/InlineContent Write path is unchanged (inline content still in tool call args)
• Confirm auto/default model selection resolves to primary model