global add: re-adding one repo deletes other repos' cross-repo edges (prune removes shared external nodes; hash gate never restores) (PE-80066) #89

Found while instrumenting Brain to query the merged global graph for PE-80066 (https://lawnstarter.atlassian.net/browse/PE-80066). Fork v8 @ 5e9d95c11837aeabbbb5d90cb45544f49dc58ca4 (CLI 0.9.40).

Summary

Re-adding one repo to the global graph deletes other repos' cross-repo edges. global add prunes a repo before re-adding it, and prune_repo_from_graph (graphify/build.py:1939-1943) removes every node carrying that tag plus every incident edge. A shared external node carries the tag of whichever repo contributed it first, so re-adding that repo destroys every other repo's edges into the shared node — and a repo whose own graph is unchanged is skipped by the hash gate and never puts them back.

Measured (12-repo merged graph)

• A rebuild that re-added 9 of 12 repos took the merged graph from 6,349 cross-repo links to 3,602.
• The demo node Lawnstarter\API\Client dropped from degree 30 to degree 5.
• Interleaving remove-then-add per repo does not fix it: that pass left partners-api with 0 of its 15 edges, because partners-api had attached to a shared node that properties-api's later removal pruned.

Workaround

Purge every tag first, then add all repos (a full rebuild). Brain's scripts/graphify/build-global-graph.py ships this as --rebuild-all; after that pass the demo node is back to degree 30 with all three contributing repos present.

Why it matters

The natural incremental workflow — "repo X changed, global add its new graph" — silently corrupts the merged graph a little more on every run, and the damage lands on repos other than the one being updated.