fix(docs): address Codex review round 2 — repoint every remaining moved-policy citation

The definitive sweep (audit every AGENTS.md mention in packages/, docs/,
examples/, scripts/) found thirteen more citations of relocated policy
and two citations of rules that never existed as quoted:

- with-key policy comments (web deepseek/perplexity e2e headers) ->
  docs/testing.md; real-impl-over-mock comments (acp harness, load,
  stream-update specs) -> docs/testing.md; defensive-pattern quotes
  (acp index.ts x3, stream-update) -> docs/defensive-patterns.md.
- md-tier repoints: real-api-e2e RFC, tool-schema-catalog RFC,
  postmortem 0001 guardrail row, adding-a-package cookbook,
  drop-bash-output-spill-files RFC, acp-subagent-backend RFC phrasing.
- Two false attributions dropped in favor of self-contained reasoning:
  tool-todo's 'don't validate scenarios that can't happen' and the
  bash-stdin-env RFC's 'Don't add features beyond what the task
  requires' (neither rule ever existed under those names).
- Citations of the two 'not golden truth' doctrines stay: those bullets
  survive verbatim in the root conventions.

Note: packages/support/ui-stdio readline TTY spec flakes under full
coverage on a heavily loaded box (passes standalone and passed the
same tree's coverage run minutes earlier); untouched by this stack.

docs/cookbook/adding-a-package.md

@@ -46,4 +46,4 @@ pnpm run test:coverage  # 100% per-file over src (types.ts exempt)
 pnpm run build && pnpm run hygiene
 ```
 
-Test expectations: every registry/registration needs an HMR-safety test (register from a child fiber, dispose it, assert cleanup). Excessive tests are welcome — see AGENTS.md.
+Test expectations: every registry/registration needs an HMR-safety test (register from a child fiber, dispose it, assert cleanup). Excessive tests are welcome — see [docs/testing.md](../testing.md).