feat: add quiet verifier output

[shreyanshjain7174]

Closes #14.

What changed:

• Adds --quiet and -q to agentgate-verify.
• Quiet text-mode success output is exactly one PASS summary line.
• Failure output remains byte-for-byte unchanged.
• --quiet is a no-op for --format json, which already returns one result object.
• Documents both automation output modes.

Verification:

• go test ./cmd/agentgate-verify -run TestRun_QuietFormat -count=1
• go test ./cmd/agentgate-verify -run TestRun_QuietDoesNotChangeJSONOutput -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

This PR adds a quiet mode to agentgate-verify so scripts can get stable, minimal success output while keeping failure output unchanged. It fits the repo’s offline receipt-chain verification tooling.

Changes:

• Added --quiet and -q flags to suppress success details in text output, leaving only the PASS: summary line.
• Added tests to assert quiet-mode text output shape, unchanged failure output, and unchanged JSON output.
• Updated README to document --quiet as the terse text-mode automation option.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
README.md	Documents --quiet / -q for terse success output in text mode.
cmd/agentgate-verify/main.go	Adds --quiet / -q flags and returns early after printing the PASS: line in text mode.
cmd/agentgate-verify/main_test.go	Adds coverage for quiet text output, unchanged failure output, and unchanged JSON output.

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