fix(cli): remove cross-platform dead symbols in startup, runtime and external editor

[1688mengdie]

Summary

Fixes #2568

Remove three cross-platform dead/unused symbols in the CLI:

• src/apps/cli/src/ui/startup.rs: delete the unused bitfun_core_types::model::ModelMutation import (pure refactoring leftover; upstream mutation handling goes through the model selection module).
• src/apps/cli/src/runtime/mod.rs: delete the unused CliRuntimeContext::agent_event_source method (its only historical consumer, the embedded app server, was removed upstream). The _agent_event_queue_owner field keeps its underscore prefix and the AgentEventSource import stays — both still have live consumers.
• src/apps/cli/src/modes/chat/external_editor.rs: split use std::ffi::{OsStr, OsString}; into #[cfg(windows)] use std::ffi::OsStr; plus an unconditional use std::ffi::OsString; (OsStr is only consumed by the Windows batch-quoting helper).

See issue #2568 for the CI run evidence with file:line and rule names.

Type and Areas

Type: bug fix (build warnings cleanup)

Areas: Rust CLI

Motivation / Impact

No direct user-facing change. Removes 1 unused import, 1 dead method, and 1 platform-scoped import warning that pollute every full CI run of main; the crate stops carrying dead symbols.

Verification

• cargo check --locked -p bitfun-cli on Windows: exit 0, the ModelMutation unused-import and agent_event_source dead_code warnings gone; no new warnings for runtime/mod.rs or external_editor.rs (the AgentEventSource import retained in runtime/mod.rs produced no unused warning, confirming it still has live consumers). The OsStr warning only fires on unix compile units, which are covered by CI.
• The unix-side effect of the external_editor.rs split (no more unused import: OsStr on ubuntu/macos compile units) depends on the CI ubuntu/macos runners: the Rust Build Check / CLI Tests jobs should confirm it (n/a locally — no Linux/macOS machine available; the platform matrix in CI is exactly the place this gets verified).

Reviewer Notes

• Three files, net −4 lines (startup −1, runtime −4, external_editor −1/+3); no behavior change: only unreachable code and an unused import are removed, plus a precise cfg scope for a Windows-only symbol.
• No #[allow] suppression is introduced.
• AI-assisted change, lightly tested (cargo check on Windows only; cross-platform behavior verified by CI).

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable. (n/a — no user-facing change)