docs: fix re-review findings — seat-migration obligation, evidence rule moved into the standard, first-cut family settled

Restore the ReactNode seats' pending-migration obligation the earlier
rewrite had flipped into a permanent exception; state the merged-PR/issue
evidence allowance in docs/AGENTS.md itself so the citations note defers
to the standard instead of overriding it; rewrite the one current-state
'first cut' fact and record the historical-stage-name exemption (with
indexical stamps still banned); pairs re-recorded.

docs/AGENTS.md

@@ -35,7 +35,7 @@ Placement: bugs → postmortems; rationale → Agent Notes; procedures → cookb
 
 ## Writing rules
 
-- **Document current state, not change history.** Avoid "previously/now/no longer", PRs, commits, and stack positions in durable prose; name the live mechanism. Put change stories in commits, PRs, Agent Notes, or postmortems.
+- **Document current state, not change history.** Avoid "previously/now/no longer", PRs, commits, and stack positions in durable prose; name the live mechanism. Put change stories in commits, PRs, Agent Notes, or postmortems; the two documentation homes may cite merged PRs and issues as evidence.
 - **Every non-trivial change includes at least one Agent Note in the same PR.** Update the owning note or add one; only mechanical/local edits are exempt ([scope](../.agents/notes/README.md#when-to-write-one)).
 - **One physical line per paragraph** (`verify-md-wrap`): use editor soft-wrap. Code blocks, tables, and list structure keep their formatting; code comments stay under the linter's column limit.
 - **Fenced `ts` blocks must compile** (`doc-typecheck`); a pasted type declaration and its original JSDoc use ` ```ts type-equiv `, while a body-stripped public class declaration uses ` ```ts public-api `; register either in the manifest so neither can drift ([mechanics](development.md#documenting-types-verbatim-ts-type-equiv)).