#21509 fix(composer): preserve draft hydration across relaunch

Summary

Preserve composer drafts across session relaunch and editor hydration by applying the handshake draft before and after editor creation.

Validation

• bun test webviews/test/composer-draft-wrap.test.ts (blocked: playwright dependency is not installed in this worktree)

This focused PR was split from the previous monolithic integration branch and is based directly on feat-cmux-next.

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Summary by cubic

Preserves composer drafts across session relaunch and editor hydration by applying the handshake draft before the editor mounts, on the mounted editor, and during session reset when the local cache is empty.

• Hydrates the editor document after creation and reapplies the draft when the session reset path is active.
• Keep hydrated draft fixtures deterministic by clearing the prompt before measuring wrap and growth.
• Aligns composer action chips and steer text sizing with the control step.