#34064 acpmux: pass result_line through bg_start_with_id so the crate compiles

feat-cmux-next does not compile acpmux. The error is E0425: cannot find value result_line in this scope at cmux-tui/crates/acpmux/src/hub/bg_tasks.rs:160, and it breaks the fleet's cmux-tui client build, so DEV builds and captures fail at base.

Cause: two merges crossed. #33317 added result_line to bg_start and to bg_run::Spec. #33779 moved the spawn into bg_start_with_id, but that function never took result_line. This PR passes it through. Workflow activities start with result_line: false, as before.

Observed / Not run

• Red on base: two fleet DEV builds (cmux-ci jobs 0ebdb48b2ed02a402df7571a and db989754dd4d6d2377916d77) fail with E0425 in acpmux.
• Green on this head (fleet): cargo check -p acpmux --all-targets, cargo clippy -p acpmux --all-targets -- -D warnings, and cargo test -p acpmux bg_ (15 passed).

Changelog

none

Agent session: agent session

🤖 Generated with Claude Code

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Summary by cubic

Fixes acpmux compilation on feat-cmux-next by passing result_line through bg_start_with_id, which previously caused an E0425 error and broke the fleet's cmux-tui client build.

• Two merges crossed: #33317 added result_line to bg_start and the spawn spec, while #33779 moved the spawn into bg_start_with_id without accepting result_line.
• Workflow activities now pass result_line: false, matching previous behavior.