Add verify-canfetch flag and git2 integration for repo accessibility checks

```
│  Agent powering down. Goodbye!
│
│  Interaction Summary
│  Session ID:                 9010bd84-1fb3-489d-ae48-89d5e7570b34
│  Tool Calls:                 26 ( ✓ 25 x 1 )
│  Success Rate:               96.2%
│  User Agreement:             96.2% (26 reviewed)
│  Code Changes:               +116 -23
│
│  Performance
│  Wall Time:                  59m 38s
│  Agent Active:               9m 53s
│    » API Time:               4m 22s (44.1%)
│    » Tool Time:              5m 31s (55.9%)
│
│
│  Model Usage                 Reqs   Input Tokens   Cache Reads  Output Tokens
│  ────────────────────────────────────────────────────────────────────────────
│  gemini-2.5-flash-lite          6         10,569             0            596
│  gemini-3-pro-preview          23        114,081       377,312         10,537
│  gemini-3-flash-preview         7         88,472        83,196            436
│
│  Savings Highlight: 460,508 (68.4%) of input tokens were served from the cache, reducing costs.
```

Cargo.toml

@@ -12,3 +12,4 @@ serde_json = "1.0"
 toml = "0.9"
 tracing = "0.1"
 tracing-subscriber = "0.3"
+git2 = { version = "0.19", features = ["vendored-libgit2"] }