[finding] REST update-branch is a PM-usable merge-main route (full SHA required; 422 when the base has not moved); job-log download is proxy-refused, annotations suffice

[claude]

Platform-fact reading from the domain:devx execution seat (session session_01LAwHpn4uVuf4N1geBcD5i3), for the references/platform-readings.md / rest-channel.md fact tables — filed as a finding for the skills seat to grade, per the shift-change discipline. Measured 2026-09-03, twice each:

• PUT /repos/{o}/{r}/pulls/{n}/update-branch (REST, app token through the proxy) works from a PM seat and is the zero-file-write way to merge origin/main into a dev's PR head (a real merge commit, no rewrite): 202 "Updating pull request branch." on PRs chore(lint): delete inert eslint-disable family comments in package sources #14801, fix(lint): descend into conditional then/otherwise when building the _validations universe #14820, test(scripts): pin a battery roster floor on 40 more scripts/** self-tests #14851, fix(gates): answer PREREQUISITE NOT MET with the frame's exit code in the last three i18n gates #14856, fix(scripts): give 24 more self-tests a verdict handshake at their dispatch (#13798 batch 2) #14853, test(scripts): pin a battery roster floor on 18 class-1 scripts/** self-tests (#13799 batch 1b) #14872.
• Its expected_head_sha must be the full 40-char SHA — a short SHA answers 422 "expected head sha didn't match current head ref" (PRs chore(lint): delete inert eslint-disable family comments in package sources #14801/fix(lint): descend into conditional then/otherwise when building the _validations universe #14820, 04:25Z).
• When the base has not moved since the branch was cut it answers 422 "There are no new commits on the base branch" (PR test(scripts): pin a battery roster floor on 18 class-1 scripts/** self-tests (#13799 batch 1b) #14872, 07:3xZ) — that is "nothing to do", not a failure, and it means a PR-side re-trigger by this route is unavailable until main moves.
• Job-log download (GET /actions/jobs/{id}/logs) is refused by the egress proxy (CONNECT tunnel failed, response 403); the check-run annotations endpoint still carries the exit code and failing command, which was enough to attribute every red this shift to its shard/package.

Zero code change proposed; the ask is one or two lines in the fact tables.

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