fix(#1441): antigravity resolver prefers GSD-owned dir over first-existing (#1442)

* fix(#1441): antigravity resolver prefers GSD-owned dir over first-existing

resolveConfigHomeFromDescriptor's dot-home-nested probe returned the first
bare-existing candidate, so a CLI user (~/.gemini/antigravity-cli) who also
had the IDE's ~/.gemini/antigravity dir was silently shadowed to the legacy
dir (probed first). Regression from #217 — pre-#217 returned
~/.gemini/antigravity unconditionally.

Add an optional probeMarker (gsd-core/VERSION) to the dot-home-nested
descriptor: a two-pass probe prefers the candidate GSD installed into, then
bare existence, then probe[0]. Behavior is byte-identical when probeMarker is
absent (windsurf etc. unaffected). Adds detectAntigravityDirAmbiguity() for
installer/operator guidance on already-misinstalled users (auto-relocation
ruled out per ADR-0008's single-configDir migration bound).

Regression tests fail before / pass after: coexistence + marker-priority
cases, end-to-end through the registry descriptor.

Claude-Session: https://claude.ai/code/session_01JU2WB23JLE3QTysPXuFJjB

* chore(#1441): add changeset for antigravity resolver fix

Claude-Session: https://claude.ai/code/session_01JU2WB23JLE3QTysPXuFJjB

capabilities/antigravity/capability.json

@@ -21,7 +21,8 @@
         "antigravity",
         "antigravity-ide",
         "antigravity-cli"
-      ]
+      ],
+      "probeExists": "gsd-core/VERSION"
     },
     "configFormat": "settings-json",
     "artifactLayout": {