docs(agents): PR-body footer rule states channel/action dependence, not a form

The edit-side sentence predicted one shape (session-URL survives, the platform
appends a bare block) that was measured on a raw REST PATCH and is false on the
MCP wrapper: an edit sent with no footer reads back with none (n=4). The
create-side sentence is folded into the same clause because the readings on
create disagree by channel too, so a rule that names only the edit action would
still invite the generalisation that produced a duplicated footer on #16694.

The sentence now states the two axes the ledger states — CHANNEL and ACTION —
keeps the imperative half, and points at
.claude/skills/pm-dispatch/references/platform-readings.md, which is where the
per-cell readings belong. The os-dev.md twin is corrected in the same register,
in place. Both files keep their line counts (AGENTS.md 1067/1068, os-dev.md
403/403); no reading is restated in either file.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P58euzUXCVJNwmhuPC9DXY

Author: claude

.claude/agents/os-dev.md

@@ -398,6 +398,6 @@ _Generated by [Claude Code](https://claude.ai/code)_                ← bare:评
 _Generated by [Claude Code](https://claude.ai/code/session_<id>)_   ← session-URL:创建 PR 正文用
 ```
 
-- session-URL 形创建用,编辑后原样存活;编辑时不带页脚发送、读回,平台在其下追加裸形块。
+- 页脚形态随通道(MCP / REST)与动作(建 / 改)变,⛔ 不由一条推其余;发你要存的,写后必回读。
 - 评论通路 MCP 与 REST 同判:整块原样存活,缺规则线则不认、再落整块留两个。
 - 耐久归属写进正文散文或评论,⛔ 不循环重贴页脚;完整读数住 AGENTS.md 同条。

AGENTS.md

@@ -425,11 +425,11 @@ _Generated by [Claude Code](https://claude.ai/code)_                 ← bare: u
 _Generated by [Claude Code](https://claude.ai/code/session_<id>)_    ← session-URL: use in PR BODIES
 ```
 
-**PR body:** `create_pull_request` keeps the session-URL form and rewrites bare into it. On EDIT
-(`update_pull_request` or a REST PATCH, measured in this repository) the session-URL footer
-survives verbatim and the platform APPENDS a bare footer block under whatever you send — so
-send an edited body with no footer of your own, read it back, and treat the appended block as
-the platform's; ⛔ never re-send a body that already carries an appended footer. Durable
+**PR body:** what a body write does to the footer depends on the CHANNEL (the MCP wrapper vs a
+raw REST call) and the ACTION (create vs edit) — measured in this repository, and the cells
+disagree, so ⛔ never generalise one to another; the readings live in pm-dispatch
+`references/platform-readings.md`. Send the footer you want stored, read the body back after
+every write, and ⛔ never re-send a body that already carries an appended footer. Durable
 attribution lives in body prose or a comment. **Issue comment:** the platform APPENDS that
 block — blank line, `---`, bare footer — and recognises only that shape (MCP
 `add_issue_comment` and direct REST agree: per-surface, not per-tool): a bare footer UNDER the