fix(tmux): background the project-accent session-created hook

[joshukraine]

Summary

• The per-project accent feature (feat(tmux): add per-project status bar accent color #207) installed its session-created hook with a foreground run-shell, which blocks tmux's command queue while the script forks bash plus several tmux subprocesses — landing on the critical path exactly as tmuxinator start spawns a project's 4–5 pane shells, adding shell-startup contention.
• This backgrounds the hook with run-shell -b so the accent work runs off the command-queue critical path. The script's own refresh-client -S still repaints the bar, so the accent continues to appear immediately on new sessions.

Changes

• tmux/.config/tmux/project-accent.tmux: add -b to the session-created hook's run-shell, and expand the inline comment to explain why it's backgrounded.

Testing

• Isolated tmux server (-L accent-test, separate from live sessions): hook definition is now run-shell -b …; a mapped session (bfo1) receives @accent = #93db29 via the backgrounded hook in ~50–100ms — no paint regression; an unmapped session falls back to the theme default.
• ./scripts/lint-shell — 42 scripts pass.
• ./scripts/run-tests — 79 bats tests pass.
• Best-effort real-world check: a morning of sequentially starting ~7 multi-pane sessions no longer produces job-queue timeout spam (validated over time in actual use, not from a single run).

Notes

• Scope: primary change only. Skipped the optional "skip the refresh-client loop when unmapped" tweak — the local accent map covers every session actually run, so the unmapped branch never fires; it would add a conditional plus a stale-paint edge case for no real benefit. Left the config-source backfill (line 27) in the foreground — out of scope and off the session-creation critical path.
• Confidence: this is the amplifier fix, not a proven root-cause fix. It removes the startup contention feat(tmux): add per-project status bar accent color #207 introduced; whether it fully ends the job-queue spam can only be confirmed across real launches. If it recurs, next steps are the abbr-queue-clear helper (feat(zsh): add abbr-queue-clear helper for a stuck zsh-abbr job queue #209) and instrumenting with JOB_QUEUE_DEBUG=1.

Closes #208