docs: recommend --lock-write over manual lockfile editing

[SisyphusZheng]

Problem

When the lockfile contains unpinned remote specifiers (e.g.
efs/heads/main), upstream pushes can cause deno install to fail with an integrity check error. The current AGENTS.md instructs contributors to manually edit the hash in deno.lock, which is error-prone and unnecessary.

Root cause

denoland/deno#32991 was closed as working as intended — Deno's integrity check is correct. deno cache --reload re-fetches content but still validates against the old integrity, so it fails. The proper approach is --lock-write, which tells Deno to accept new content and update the lockfile.

Solution

Replace the manual hash editing instruction in AGENTS.md with the deno install --lock-write command, which is Deno's official recommended fix.