[Bug]: unused import / dead method warnings in CLI startup.rs, runtime/mod.rs and external_editor.rs

[1688mengdie]

Summary

The upstream CI logs for main report three cross-platform unused-import/dead-code warnings in the CLI. Evidence from GitHub Actions run 33035362609 (reproduced on ubuntu/macos/windows compilation units and across runs 33050974141 and 33047041028):

• warning: unused import: bitfun_core_types::model::ModelMutation --> src/apps/cli/src/ui/startup.rs:33:5
• warning: method agent_event_source is never used --> src/apps/cli/src/runtime/mod.rs:121:19
• warning: unused import: OsStr --> src/apps/cli/src/modes/chat/external_editor.rs:1:16

Cross-checking the crate:

• The ModelMutation import in ui/startup.rs has no consumer in that file; upstream mutation handling goes through the model selection module. It is pure refactoring leftover.
• CliRuntimeContext::agent_event_source has no caller left anywhere in the crate: its only historical consumer (the embedded app server) was removed upstream.
• In external_editor.rs, OsStr is consumed only by the Windows batch-quoting helper, while OsString is used unconditionally; the combined use std::ffi::{OsStr, OsString}; therefore triggers unused import: OsStr on non-Windows builds.

Root Cause Analysis

• One leftover import after a refactor, one accessor whose only consumer was removed upstream, and one combined import mixing a Windows-only symbol with a cross-platform symbol. Each of the three warnings is structural and reproduced by every full CI run of main (runs 33035362609 / 33050974141 / 33047041028).

Proposed Fix

• Delete the unused ModelMutation import line in ui/startup.rs.
• Delete the unused agent_event_source method in runtime/mod.rs (keep the underscore-prefixed _agent_event_queue_owner field and the AgentEventSource import, which still have live consumers).
• Split the import in external_editor.rs into #[cfg(windows)] use std::ffi::OsStr; plus an unconditional use std::ffi::OsString;.

No #[allow] suppression is involved; the dead symbols are removed and the platform-scoped import is scoped precisely.