* fix(#2018): applySurface empty manifest no longer deletes gsd-* agents
The agent-prune loop in _syncGsdDir deleted any gsd-*.md not in the staged
set. When the manifest resolved empty (null/non-object, no entries, no files
key, or unresolvable install source root), the staged set was empty → every
gsd-* agent was deleted. Skills were guarded by pruneSkillDirs' manifest-
membership check; agents had no equivalent.
- src/surface.cts: skip the agent-prune loop when the manifest is empty/absent
(copy still runs — genuinely new agents are added).
- tests/surface-empty-manifest-agents.test.cjs: boundary matrix — empty
manifest preserves agents (Map() + undefined); non-empty manifest + empty
staged still prunes (boundary); empty manifest + new staged copies new +
preserves existing.
Closes#2018
* docs(#2018): backfill changeset pr 2031
