* test(#3163): phase add must insert in the active milestone, not the trailing archive
Regression for #3163: cmdPhaseAdd/cmdPhaseAddBatch pick the insertion point
via rawContent.lastIndexOf('\n---'), the file's last horizontal rule — which
on a roadmap with shipped/history material after the active phase list sits
deep in archive. Rows 1/2/4 fail RED on next (entry lands after the archive
heading); row 3 guards the no-milestone legacy fallback.
* fix(#3163): scope phase.add insertion to the current milestone window
cmdPhaseAdd and cmdPhaseAddBatch picked the insertion point via
rawContent.lastIndexOf('\n---') — the file's last horizontal rule, which on
a roadmap with shipped/history material after the active phase list sits deep
in archive. Extract phaseEntryInsertOffset(rawContent, cwd): scope the search
to currentMilestoneRawRanges' primary window so the entry lands at the end of
the active phase list. Fall back to the legacy whole-file heuristic when no
current milestone resolves, preserving simple no-milestone roadmaps. Applies
to both cmdPhaseAdd and cmdPhaseAddBatch (identical expression); the decimal
insert path was already header-anchored and is untouched.
* docs(#3163): add changeset
* docs(#3163): backfill changeset PR number (3400)
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Co-authored-by: sim <sim@local>
