docs(agents): fix dangling Tier 3 reference

Line 132 referenced a Tier 3 heading that no longer exists in AGENTS.md;
that content moved to philosophy-appendix.md when the doc was split for
the PR. Point the reference there instead.

Signed-off-by: Joe Julian <me@joejulian.name>

AGENTS.md

@@ -129,7 +129,7 @@ In practice:
 
 For an agent reviewing a PR, this means: raise these principles as prompts for a human reviewer and as guidance for the author, never as automated rejections. Tone is not a nicety here. It is the difference between review that helps and review that drives contributors away.
 
-The Tier 3 narratives (the values-templating and post-render refusals, the automated-review discussion) show this posture in maintainers' own words.
+The field notes in `philosophy-appendix.md` (the values-templating and post-render refusals, the automated-review discussion) show this posture in maintainers' own words.
 
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