* test(#2639): add regression for local-ahead-of-origin warning in handle_branching
execute-phase.md's handle_branching forks from origin/$DEFAULT_BRANCH. When
local is ahead (unpushed commits), the phase branch silently misses them.
The test asserts the workflow checks for local-ahead-of-origin and warns.
* fix(#2639): warn when local is ahead of origin before forking phase branch
handle_branching forks the phase branch from origin/$DEFAULT_BRANCH. When
local $DEFAULT_BRANCH is ahead (unpushed commits like plan/research docs),
the fork silently misses those commits. Now a loud WARNING is printed to
stderr naming the commit count and advising the user, matching the existing
uncommitted-changes warning pattern.
* chore(#2639): add changeset fragment
* fix(#2639): condense warning under ADR-857 cap + merge emitted-drift ack
gsd-test gate caught: (1) execute-phase.md exceeded the 93600-byte Phase 6
ceiling — condensed the WARNING from 3 echo lines to 1. (2) emitted-attribution
flagged the growth without an ack — merged into the existing #2930 ack fragment
(execute-phase.md was already acked there; can't have two acks for the same path).
* fix(#2639): condense warning further to clear the 93400 comfortable-margin gate
The ADR-857 Phase 6 test has two assertions: <93600 (hard ceiling) and
<=93400 (comfortable margin). Condensed from 3 lines to 2 to fit under
93400 (now 93369).
* chore(#2639): backfill changeset PR number 2981
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Co-authored-by: sim <sim@local>
6c96b13cfe