docs: tmux-agents v1.7.3 lessons from shipping
I shipped v1.7.3 of tmux-agents, rewriting AGENTS.md around what the agents learned shipping v1.6 and v1.7. It now covers running tests, treating flaky tests as release blockers, the list-build process budget, working in a shared checkout and recording decisions before implementation. The release routine says to reply as soon as a release is out and never move a published tag, while the DESIGN.md overview now matches current behaviour.



