docs(agents): fix stale mise task and rendering pipeline description

[plttn]

Summary

• Removes the mise run all line from AGENTS.md's Commands section — that task was deleted in chore(mise): remove all task #88, so the doc pointed at a command that no longer exists. CONTRIBUTING.md got the matching fix in docs(contributing): document mise tasks individually #89; AGENTS.md was missed.
• Updates the Rendering pipeline section to match the current mechanism after fix(prompt): stop a stale render sticking on the prompt #94: the background job now writes to a per-pid scratch tmpfile and signals the parent with SIGUSR1, instead of writing a universal variable that an --on-variable handler watched. Also notes the SIGUSR1-unavailable fallback to synchronous rendering, which wasn't documented at all.

Test plan

• Read AGENTS.md against current mise.toml and functions/fish_prompt.fish to confirm accuracy