docs(agents): record lessons from upstreaming the debug-relocation work

[zackees]

Adds an "Upstream contributions: lessons learned" section to AGENTS.md, plus a branch convention for up/* branches. It records what reduced friction, or would have, with wild-linker#2620, wild-linker#2621, wild-linker#2628, wild-linker#2629 and wild-linker#2630:

• how the maintainers weigh complexity, and how to keep optimizations self-contained;
• measurement: ablation, a small-link regression check, re-measuring after a rebase, and perf annotate findings;
• PR writing, following upstream's CONTRIBUTING.md, and review etiquette;
• upstream CI gotchas (wasip1 has no temp dir, RISC-V/LoongArch relocations read the bytes they patch);
• fork benchmarking and GitHub mechanics: bench base branches, label timing, the API rate limit;
• local environment lessons, including the save-dir toolchain truncation.

Docs only.