feat: add JSON verifier output

[shreyanshjain7174]

Closes #13.

What changed:

• Adds --format text|json to agentgate-verify, retaining text as the default.
• JSON output is exactly one safe result object with verification counts, result state, stable mismatch reason, full head hash on success, completeness, and export range when present.
• Preserves exit codes: 0 pass, 1 mismatch, 2 input/config error.
• Documents --format json beside the standalone verifier command.

Verification:

• go test ./cmd/agentgate-verify -race -count=1
• go test ./... -race -count=1
• go vet ./...
• gofmt -l cmd/agentgate-verify/main.go cmd/agentgate-verify/main_test.go
• git diff --check

All passed locally.

[Copilot]

Pull request overview

Adds a machine-readable JSON output mode to agentgate-verify. This helps CI and auditor tooling consume verification results without parsing text.

Changes:

• Adds --format text|json flag to cmd/agentgate-verify and emits a single JSON result object on stdout when json is selected.
• Documents --format json in the README verifier section.
• Adds tests for JSON output, default text equivalence, and invalid format handling.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
README.md	Mentions --format json for script-friendly verifier output.
cmd/agentgate-verify/main.go	Adds --format flag, JSON result struct, and JSON encoder output path.
cmd/agentgate-verify/main_test.go	Adds coverage for JSON output success and mismatch cases, plus format validation.

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