♻️ Single-home the actionable rules and the rationale

development/ had restated the actionables that CONTRIBUTING.md owns: the
branching step list, the script prefix list, the feedback-tier rule of thumb,
the commit convention and the type-driven test loop. Those now live only in
CONTRIBUTING.md; development/ keeps the decision blocks and links to the rule.
development/README.md, CONTRIBUTING.md and AGENTS.md state the 'write each fact
once' principle explicitly.

development/README.md

@@ -10,6 +10,9 @@ rules: read the relevant file here before changing an area, and update it when
 a decision changes, rather than leaving an obsolete reason behind. A rule and
 its reason drift apart when they live in one place and are maintained in two;
 keeping the rule in CONTRIBUTING.md and the reason here keeps each single-homed.
+**Each fact is written once**: the actionable rule in CONTRIBUTING.md, the
+reason here. Neither restates the other — when the same fact would be useful in
+both, one links to the other instead of copying it.
 
 User-facing documentation lives in [README.md](../README.md). A `docs/` folder
 is deliberately not used yet: the convention is that `docs/` is the future