Print helpful message when vouch init is run without --domain

[rampyg]

Hi @fahadhewad, re-opening your contribution. Your original PR #132 was closed automatically due to a maintenance issue on the main branch, nothing on your side. Your branch and commits are unchanged. Apologies for the disruption, and thank you. The earlier review comments did not carry over, so please continue here.

Original PR: #132

---

Closes #128.

When vouch init is run with no domain argument, the CLI silently falls back to example.com with no explanation. This change prints a short message to stderr telling the user what default was chosen and how to pass their own domain.

Example output with this fix applied:

vouch init
No domain specified, using example.com. Pass --domain your.domain to set a real domain.

[github-actions]

Vouch Verified Contributor ✨

Thank you @fahadhewad. This merged pull request earns a signed
Vouch Verified Contributor certificate, chained to the project root authority.

Your certificate (download and share it): https://vouch-protocol.com/c/fahadhewad/182

Add the badge to your profile or site (optional):

[![Vouch Verified Contributor](https://img.shields.io/badge/Vouch-Verified_Contributor-7C2D3A?style=for-the-badge&labelColor=2d2d2d)](https://vouch-protocol.com/c/fahadhewad/182)

Your Verifiable Credential (eddsa-jcs-2022)

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    "https://vouch-protocol.com/contexts/v1"
  ],
  "id": "urn:uuid:1b6b9507-42b5-4e1a-8aa5-5ca1b1f979e5",
  "type": [
    "VerifiableCredential",
    "VouchCredential"
  ],
  "issuer": "did:web:vouch-protocol.com:contributors",
  "validFrom": "2026-07-04T09:31:58Z",
  "validUntil": "2126-06-10T09:31:58Z",
  "credentialSubject": {
    "id": "did:web:vouch-protocol.com:contributors",
    "vouchVersion": "1.0",
    "intent": {
      "action": "attest",
      "target": "github:fahadhewad",
      "resource": "https://github.com/vouch-protocol/vouch/pull/182",
      "role": "verified-contributor",
      "repository": "vouch-protocol/vouch",
      "pullRequest": 182
    },
    "delegationChain": [
      {
        "issuer": "did:web:vouch-protocol.com",
        "subject": "did:web:vouch-protocol.com:contributors",
        "intent": {
          "action": "attest",
          "target": "github:fahadhewad",
          "resource": "https://github.com/vouch-protocol/vouch/pull/182",
          "role": "verified-contributor",
          "repository": "vouch-protocol/vouch",
          "pullRequest": 182
        },
        "validFrom": "2026-06-19T04:45:43Z",
        "validUntil": "2036-06-16T04:45:43Z",
        "parentProofValue": "z5g1N4udQdy5asdxnV3qFhApB5cAh28DkMqHjEUSNG6cc8hrDDsxWyVbgn1XXUFR"
      }
    ]
  },
  "proof": {
    "type": "DataIntegrityProof",
    "cryptosuite": "eddsa-jcs-2022",
    "created": "2026-07-04T09:31:58Z",
    "verificationMethod": "did:web:vouch-protocol.com:contributors#key-1",
    "proofPurpose": "assertionMethod",
    "proofValue": "z2va7zteZKtSSJJjEPxFutDp4R7R6QhsnEWWVWiXnVZP7hmUs5Lrqyvf6pMBE4pcaTCv3jEX4Z7dDC8iHEkBitcEa"
  }
}

[github-actions]

Vouch Verified Contributor ✨

Thank you @fahadhewad. This merged pull request earns a signed
Vouch Verified Contributor certificate, chained to the project root authority.

Your certificate (download and share it): https://vouch-protocol.com/c/fahadhewad/182

Add the badge to your profile or site (optional):

[![Vouch Verified Contributor](https://img.shields.io/badge/Vouch-Verified_Contributor-7C2D3A?style=for-the-badge&labelColor=2d2d2d)](https://vouch-protocol.com/c/fahadhewad/182)

Your Verifiable Credential (eddsa-jcs-2022)

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    "https://vouch-protocol.com/contexts/v1"
  ],
  "id": "urn:uuid:f5679aab-e0b7-4e51-82cd-63e740e5ee46",
  "type": [
    "VerifiableCredential",
    "VouchCredential"
  ],
  "issuer": "did:web:vouch-protocol.com:contributors",
  "validFrom": "2026-07-04T09:34:53Z",
  "validUntil": "2126-06-10T09:34:53Z",
  "credentialSubject": {
    "id": "did:web:vouch-protocol.com:contributors",
    "vouchVersion": "1.0",
    "intent": {
      "action": "attest",
      "target": "github:fahadhewad",
      "resource": "https://github.com/vouch-protocol/vouch/pull/182",
      "role": "verified-contributor",
      "repository": "vouch-protocol/vouch",
      "pullRequest": 182
    },
    "delegationChain": [
      {
        "issuer": "did:web:vouch-protocol.com",
        "subject": "did:web:vouch-protocol.com:contributors",
        "intent": {
          "action": "attest",
          "target": "github:fahadhewad",
          "resource": "https://github.com/vouch-protocol/vouch/pull/182",
          "role": "verified-contributor",
          "repository": "vouch-protocol/vouch",
          "pullRequest": 182
        },
        "validFrom": "2026-06-19T04:45:43Z",
        "validUntil": "2036-06-16T04:45:43Z",
        "parentProofValue": "z5g1N4udQdy5asdxnV3qFhApB5cAh28DkMqHjEUSNG6cc8hrDDsxWyVbgn1XXUFR"
      }
    ]
  },
  "proof": {
    "type": "DataIntegrityProof",
    "cryptosuite": "eddsa-jcs-2022",
    "created": "2026-07-04T09:34:53Z",
    "verificationMethod": "did:web:vouch-protocol.com:contributors#key-1",
    "proofPurpose": "assertionMethod",
    "proofValue": "z5Yr94GR34JEFgu4ZN26fXssxBLLrnKCdgwJ2XeHhrrK3aeQRyVAUPP2rjATHuEj39yQncetnmfEmrZmCzZkDc6ju"
  }
}