docs: give docs/design/ its own AGENTS.md

[derek73]

Documentation only. Splits the rules-documentation conventions out of
root AGENTS.md into a nested docs/design/AGENTS.md.

Why, and the sorting rule

Root AGENTS.md is loaded into every session at ~28,300 tokens
regardless of what that session is doing; docs/design/ is read on
demand. So the question for each convention isn't where it belongs
topically — it's whether a session that never touches that area
still needs it.

Claude Code loads a nested AGENTS.md when a session reads or edits
anything in that subtree, so the moved conventions arrive exactly when
relevant and cost nothing otherwise. That also closes a gap a plain
doc plus a "see also" would leave: the review axes only work if
someone remembers to look, and the thing that lost §5 for a month was
exactly a pointer nobody followed.

root AGENTS.md          105,755 → 97,416 chars   (~2,254 tokens/session)
docs/design/AGENTS.md             8,274 chars

What moved

Fires only when working on these documents:

• Landing a design (the spec-residue distillation checklist)
• A count in a dated entry is evidence, not a live fact
• Primary-source review
• Reviewing a docs/design change: the nine axes

What stayed, and why that's the interesting half

Six conventions stay in root because their trigger is elsewhere:

• Same-PR amendment rule — entirely about rules.md, and still
  belongs in root, because the session that needs it is editing Python
  and would never load a file under docs/design/. Sorting by subject
  would have moved it and quietly broken the rule that keeps docs and
  code in step.
• Counting claims — prose anywhere: tests, release notes, comments.
• Release-log claims, backlog triage, markdown line width
  (it names root AGENTS.md itself), guard-test negative controls.

Over half the section by weight had to stay, which is why the saving
is 2,254 tokens rather than the 3,500 the section's size suggested.

Verification

• Lossless: concatenating the two files and comparing the word
  multiset against the original leaves zero words missing.
• uv run --frozen pytest — 3459 passed, 20 skipped, 11 xfailed.
• The pointer left in root is imperative, not "see also", and tells
  you to read the file directly if your tool doesn't do nested
  discovery — AGENTS.md is a cross-tool convention and only Claude
  Code's recursive loading is confirmed here.

docs/design/AGENTS.md is now itself subject to the nine axes it
contains.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.51%. Comparing base (5351289) to head (b03ec4c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #389   +/-   ##
=======================================
  Coverage   98.51%   98.51%           
=======================================
  Files          44       44           
  Lines        2895     2895           
=======================================
  Hits         2852     2852           
  Misses         43       43           

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