Refactor ImixAgent task output to use bounded channel

[KCarretto]

Refactor ImixAgent to use std::sync::mpsc::sync_channel for task output buffering instead of a Vec. This introduces backpressure (via bounded channel) and safe concurrency handling. flush_outputs now aggregates multiple output messages for the same task into a single report.

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PR created automatically by Jules for task 13824914665251077122 started by @KCarretto

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[github-actions]

Summary

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
2558    ±0	2558    ±0	0    ±0	0    ±0	0    ±0	0    ±0	1ms    ±0

Previous Results

Build 🏗️	Result 🧪	Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
#861	✅	2558	2558	0	0	0	0	20.2s

Insights

Average Tests per Run	Total Flaky Tests	Total Failed	Slowest Test (p95)
2558	0	0	5.2s

Slowest Tests

Test 📝	Results 📊	Duration (avg) ⏱️	Duration (p95) ⏱️
eldritch-libsys: std::dll_inject_impl::tests::test_dll_inject_simple	1	5.2s	5.2s
imix::bin/imix: install::tests::test_install_execution	3	1.2s	3.4s
imix::bin/imix: install::tests::test_install_execution	3	1.2s	3.4s
imix::bin/imix: install::tests::test_install_execution	3	1.2s	3.4s
imix::bin/imix: tests::task_tests::test_task_streaming_error	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_error	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_error	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s

_{🎉 No failed tests in this run. | 🍂 No flaky tests in this run.}

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