agentgate-verify: add a --format json output option

[shreyanshjain7174]

What

Add a --format json flag to cmd/agentgate-verify that emits the verification result as a single machine-readable JSON object on stdout, instead of (or as an alternative to) the current human-readable text lines.

Why

Today agentgate-verify's output (PASS: %d receipts verified, head seq=%d hash=%x, completeness: ..., range: ..., or FAIL: seq=%d reason=%s (...)) is designed to be read by a person, not parsed by a script or CI pipeline. A --format json option lets an auditor's tooling consume the result programmatically without scraping text.

Files to touch

• cmd/agentgate-verify/main.go — add a --format flag (text default, json alternative) to the run() function's flag set; when json is selected, marshal a small result struct instead of the current fmt.Fprintf calls.
• cmd/agentgate-verify/main_test.go — add test coverage for the new flag.

Suggested JSON shape

Something close to:

{
  "ok": true,
  "verified_count": 5,
  "head_seq": 5,
  "head_entry_hash": "a1b2c3...",
  "complete": true,
  "reason": "",
  "range": "full"
}

For a failure:

{
  "ok": false,
  "failed_at_seq": 3,
  "reason": "entry_hash_mismatch",
  "verified_count": 2,
  "total_receipts": 5
}

Match the existing receipt.VerifyResult struct's fields (internal/receipt/verifier.go) rather than inventing new field names where one already exists.

Exit codes must not change: 0 pass, 1 chain/signature mismatch, 2 I/O or config error — the same rule applies whether --format text or --format json is selected.

Acceptance checks

• agentgate-verify --format json (with a valid, passing chain) exits 0 and prints one valid JSON object with "ok": true.
• agentgate-verify --format json against a tampered chain exits 1 and prints one valid JSON object with "ok": false and the correct "reason".
• The default (no --format flag, or --format text) behavior is byte-for-byte unchanged from today.
• --format with an unrecognized value exits 2 with a clear stderr message, matching the existing pattern for other flag validation errors in run().

Test path (no secrets needed)

go test ./cmd/agentgate-verify/... -race -count=1 -v

The existing test helpers (buildTestChain, writeJSONLFile) already build real signed receipt chains in a temp SQLite database — no live AgentGate instance, network access, or secrets are needed.