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.

packages/todo/tool-todo/src/index.ts

@@ -51,8 +51,8 @@ const DESCRIPTION =
  * `InferArgs` maps an `enum` string prop to plain `string`, so the compiler sees
  * `args.todos` as `{ content: string; status: string }[]`; the
  * `status as TodoItem['status']` narrowing records that registry guarantee
- * rather than re-checking it (an unreachable re-check would be dead code — see
- * AGENTS.md "don't validate scenarios that can't happen"). What remains is the
+ * rather than re-checking it (an unreachable re-check would be dead code the
+ * coverage gate would flag). What remains is the
  * value rules the DSL has no vocabulary for: non-empty unique content (stored
  * trimmed, so the persisted value matches the dedupe/length key), and at most
  * one `in_progress` task.