fix: remove duplicate -o json flag in gemini CLI config

Fixes #291

The gemini CLI configuration had a duplicate -o json flag that caused
Gemini to return plain text instead of JSON, breaking the clink parser.

Root cause:
- Internal defaults (clink/constants.py) already provide -o json
- Config file (conf/cli_clients/gemini.json) duplicated this flag
- Result: command became 'gemini -o json ... -o json' (duplicate)

This caused Gemini CLI to malfunction and output plain text, leading to
'Failed to decode Gemini CLI JSON output' errors.

Solution:
Remove the duplicate -o json from additional_args since it's already
provided by internal defaults, matching the pattern used by claude and
codex CLI configurations.

After fix:
- Command: gemini -o json --telemetry false --yolo (correct)
- Output: Valid JSON with response and stats
- Parser: Successfully extracts content and metadata

conf/cli_clients/gemini.json

@@ -4,9 +4,7 @@
   "additional_args": [
     "--telemetry",
     "false",
-    "--yolo",
-    "-o",
-    "json"
+    "--yolo"
   ],
   "env": {},
   "roles": {