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

Two author-side rules the fleet states and this file did not, verbatim from
obin-ai/handbook#313 (merged 2026-10-13): prove reuse before you build, and
build the simplest thing that works — a ~500 hand-written line budget, 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.

The wording is byte-identical across every repo in this sweep; only the list
marker follows this file's own style. Docs only: no code, no gate, no
behaviour change.

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

Author: antoinetoussaint-byte

AGENTS.md

@@ -70,6 +70,22 @@ These are the fleet's rules, not this repo's, and they are stated here because
 an agent reads this file and not the tracker:
 https://github.com/obin-ai/handbook/issues/68.
 
+- **Prove reuse before you build.** The default answer is that the platform
+  already has it and your change fills a gap or adds a helper on it, so the PR
+  body names what you reused and what you deliberately did not. A second
+  mechanism for a job one seam already does is refused however clean it is. Two
+  tells you are reinventing: what you are writing re-answers a question an
+  existing seam answered, and your reason for its placement is that a
+  convenient loop was already there rather than that the responsibility belongs
+  to it.
+- **Build the simplest thing that works; size is the signal.** A coding task's
+  budget is about **500 hand-written lines** — generated output does not count,
+  tests count apart — and past **2,000** the change has become a second system,
+  so **stop and ask the user** instead of finishing it. Two tells, checkable
+  before the code exists: you cannot name the one responsibility the change
+  adds, and it is growing to make the end goal pass rather than to put a
+  behaviour where it belongs. Wanting it to work licenses no means to get
+  there.
 - **A gap in the tooling is a bug in the tooling** — never a reason to reach
   around it. Not as a "workaround", not "just this once", not "until the verb
   lands". A missing capability in `codefly`, `buf` or the generation scripts is