fix(exec): center header, fix stuck timer, brighten command preview

[avihut]

Summary

Three rendering fixes to the live UI of daft exec against multiple worktrees:

• Header centering — the scope-summary line previously used a fixed 8-char prefix, leaving the summary visibly left-of-center. Prefix/suffix dashes are now balanced so the summary sits in the middle of the 60-column divider.
• Stuck spinner timer — the elapsed-time suffix was promoted onto the spinner only inside update_job_output, so a long-running silent job (Compiling daft … for tens of seconds with no fresh stdout) never received the [HH:MM:SS] suffix. Promotion is now driven by a per-job one-shot thread that fires on a wall-clock deadline.
• Readable command preview — the in-flight ❯ <cmd> rendered in DARK_GREY (palette 240), which is barely distinguishable from the background on dark themes. The color is dropped so the in-flight preview matches the post-run rows, which already use the default foreground.

Test plan

• mise run fmt:check
• mise run clippy
• mise run test:unit — 1179 passed
• mise run test:manual --ci — 440 scenarios / 1561 steps passed
• New regression tests:
  • format_header_line centering invariant + total-width preservation + overflow-summary fallback
  • timer_promotes_on_wall_clock_even_without_output — proves the timer suffix appears even when the job emits no stdout past the threshold
• Manual visual check of daft exec --all -- mise dev against ≥2 worktrees on a slow build

🤖 Generated with Claude Code