Keep template AI guidance optional by default

README now explains that AGENTS.md and CLAUDE.md are optional additions for downstream projects rather than opinionated defaults shipped by the template.

Constraint: The template should stay neutral and avoid prescribing AI workflow files for every downstream repo
Rejected: Remove AI guidance mentions entirely | downstream users would lose the cue that they can add project-specific guidance later
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep template defaults neutral and add AI guidance files only in downstream projects that want them
Tested: make check
Not-tested: Post-merge repository automation beyond the already green PR checks

Author: yujeongdev

README.md

@@ -59,6 +59,11 @@ Let's build something fun, Yujeong.
 └── uv.lock
 ```
 
+## Optional AI guidance files
+
+If your real project wants agent-specific instructions, add `AGENTS.md` and/or `CLAUDE.md` later
+with project-specific guidance. This template does not ship opinionated defaults for them.
+
 ## Quality defaults
 
 This template is intentionally strict by default: