fix: grow hook output window dynamically instead of pre-allocating blank lines

[avihut]

Summary

• Remove pre-allocation of separator and tail ProgressBar slots in start_job — no blank lines appear before any output arrives
• Add lazy growth in update_job_output: window starts at 0 lines and grows one slot per output line until the configured maximum (default 6), then rolls
• Uses a 4-phase borrow-split pattern to work around Rust's aliasing rules between self.jobs and self.mp

Test Plan

• test_dynamic_window_starts_empty — tail count is 0 after start_job with no output
• test_dynamic_window_grows_with_output — count increments 1 per line up to max
• test_dynamic_window_caps_at_max — count stays at max after cap; buffer holds all lines
• test_dynamic_window_zero_output_no_separator — silent jobs finish cleanly with no bars allocated
• All 350 unit + 179 integration tests pass

Fixes #236

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