fusion/.changeset/fn-4624-worktrunk-auto-install.md

@runfusion/fusion

@runfusion/fusion
minor

Auto-install flow for the optional worktrunk worktree backend: when worktrunk.enabled is on and the binary is missing, Fusion downloads a SHA-256-verified pinned release (v0.4.2) into ~/.fusion/bin/ with a cargo install fallback under network_api approval policy. Pinned release supports darwin-arm64, darwin-x64, linux-x64, linux-arm64; Windows falls back to cargo-only. Install attempts emit run-audit binary:install-* events.