docs(AGENTS): rewrite the root standing orders to the 1,500-word budget

Applies the documentation standard to its biggest offender. Every rule
survives as one to three lines plus a link to its durable home; the
stories, duplicate statements, and re-narrations go:

- Situational clusters evict to new homes: docs/testing.md (tiers,
  with-key policy, real-over-mock, world-verification, real-entry-path
  guards), docs/defensive-patterns.md (the bug-class rules), and
  docs/cookbook/responding-to-pr-review-on-a-stack.md (the stacked-PR
  review procedure).
- Doc-authoring rules consolidate into docs/AGENTS.md § Writing rules
  (current-state-never-history, md-wrap, ts-block compilation, @mode,
  catalog same-change, pair same-change).
- packages/README.md drops to the group table + the extension-vs-bundle
  dependency rule; the hand ASCII graph yields to the generated
  module-graph.md; group READMEs are the canonical per-package map.
- packages/AGENTS.md keeps only its packages-specific rules (export
  shape, ctx.get, real-Loader coverage); examples/AGENTS.md repoints
  its with-key-policy link; rfc/README.md loses a narrated-history
  aside; dsh-code-review / dsh-find-simplifications / verify-md-wrap
  references follow the moved content.
- Budget manifest ratchets: AGENTS.md 8200 -> 1500 (now 1,495 words),
  packages/README.md 1900 -> 600, packages/AGENTS.md 600 -> 450; the
  two new eviction docs join the budget set (testing 800, defensive
  550); docs/AGENTS.md raises 1000 -> 1250 for the absorbed writing
  rules (the one justified increase). The doc-tiers RFC's deferred list
  prunes the two items this change ships.

docs/rfc/README.md

@@ -1,6 +1,6 @@
 # RFCs
 
-One kind of design doc lives here. An **RFC** records a decision or proposal that shapes this codebase — the *why* and *what we gave up*, the parts code and docs can't carry. (Earlier this split into separate "ADR" and "RFC" trees; they were unified, since most ADRs were simply implemented RFCs.)
+One kind of design doc lives here. An **RFC** records a decision or proposal that shapes this codebase — the *why* and *what we gave up*, the parts code and docs can't carry.
 
 ## Layout and naming