docs: carry the fleet-wide reuse and size-budget rules into AGENTS.md

From obin-ai/handbook#313 (merged 2026-10-13): prove reuse before you build,
and build the simplest thing that works — a budget of about 500 hand-written
lines, where past 2,000 the change has become a second system and the agent
stops and asks the person who wanted it instead of finishing it.

This file is 186 of the 200 lines its own stated cap allows, which the full
fifteen-line rule would have broken (`conformance_test.go` caught it), so the
rules land in one bullet with both headlines and both numbers and the full text
one link away. Nothing was cut; the file is 192 lines.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: antoinetoussaint-byte

AGENTS.md

@@ -28,6 +28,11 @@ a user's Python process: nearly every defect is observed as "the service does
 not come up", and the local substitute for whatever did not resolve is always
 within reach.
 
+- **Prove reuse before you build**, and **build the simplest thing that works;
+  size is the signal**: fill a gap on what the platform already has, keep a
+  coding task to about **500 hand-written lines**, and past **2,000** **stop and
+  ask the user**. Both in full, with their tells, in [the
+  handbook](https://github.com/obin-ai/handbook/blob/main/AGENTS.md).
 - **A gap in the tooling is a bug in the tooling — never a reason to reach
   around it.** When something needs a step `codefly`, `uv` or a companion image
   does not perform, the answer is a capability fixed in whichever tool owns it,