package: pin gaxios override to ^6.7.1 (required by googleapis-common)

Previous override (gaxios: 7.1.4) was set in 5c64f991b to silence a
glob@10 deprecation warning. That choice is incompatible with
@google/gemini-cli-core's dependency graph: googleapis-common@7.2.0
does `require("gaxios/build/src/common")` — a deep internal path that
gaxios 6.x exposed but 7.x tightened out of its exports field.

Swapping to ^6.7.1 restores cli-core's runtime: a probe using the
installed cli-core + the user's cached ~/.gemini/oauth_creds.json now
successfully reaches https://cloudcode-pa.googleapis.com/v1internal:
streamGenerateContent and gets a real response from gemini-2.5-flash.

The glob deprecation the previous override fixed is cosmetic and
doesn't block anything. Live cli-core functionality trumps npm warning
noise.

Unblocks task #3: replacing the handwritten fetch() transport in
pi-ai/src/providers/google-gemini-cli.ts with CodeAssistServer calls.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -156,6 +156,6 @@
     "koffi": "^2.9.0"
   },
   "overrides": {
-    "gaxios": "7.1.4"
+    "gaxios": "^6.7.1"
   }
 }