feat(doc-standards): documentation tiers, budgets, and the ceiling gate

Standing docs accrete a paragraph per PR with nothing pushing back; the
root AGENTS.md reached 8,130 words in 50 commits with the same rule
stated two and three times. This encodes the counter-pressure:

- docs/AGENTS.md becomes the documentation standard: the tier taxonomy
  (one home per fact), target word budgets, and the slop checklist.
- verify-doc-budgets joins doc-sync: word ceilings for the six
  accretion-prone standing docs, manifest-driven, frozen at current
  sizes and ratcheted down as each doc is brought to target.
- .agents/skills/dsh-doc-standards: the thin placement/audit/red-gate
  workflow over the standard, mirroring the dsh-translate-docs split.
- RFC (implemented/process) records the decision, alternatives, and
  the first audit cycle's deferred work list.

The gate's first catch was the standard itself (1,057 > 1,000); it
ships condensed to 984 words rather than with a raised ceiling.

scripts/doc-budgets.manifest.json

@@ -0,0 +1,8 @@
+{
+  "AGENTS.md": 8200,
+  "docs/AGENTS.md": 1000,
+  "docs/architecture.md": 3950,
+  "examples/AGENTS.md": 600,
+  "packages/AGENTS.md": 600,
+  "packages/README.md": 1900
+}