feat: add Hunter.io integration (closes #14)

[jackulau]

Summary

Adds the Hunter.io integration to @jam-nodes/nodes with three operations for email discovery and verification, plus an API key credential definition. Follows the established defineNode() pattern used by existing integrations (Airtable, Slack, Apollo).

Operations

• hunterDomainSearchNode — searches all email addresses associated with a domain. Supports optional filters for limit (1-100), type (personal/generic), seniority (junior/senior/executive), and department (sales/marketing/hr/it/finance/executive). Calls GET https://api.hunter.io/v2/domain-search.
• hunterEmailFinderNode — finds the email address of a specific person at a domain given their first and last name. Returns email, confidence score, position, and company. Calls GET https://api.hunter.io/v2/email-finder.
• hunterEmailVerifierNode — verifies the deliverability of an email address. Returns status, score, and 8 boolean verification fields (regexp, gibberish, disposable, webmail, mxRecords, smtpServer, smtpCheck, acceptAll). Calls GET https://api.hunter.io/v2/email-verifier.

Design

• API key authentication via query parameter (?api_key=...), using defineApiKeyCredential with authenticate.type: 'query'
• URLSearchParams for all query string construction (handles URL-encoding of special characters in names like O'Brien)
• Snake_case API response fields mapped to camelCase outputs per-field (e.g., first_name → firstName, mx_records → mxRecords, extracted_on → extractedOn)
• Zod z.enum() validation on constrained input fields (type, seniority, department) to catch invalid values before calling the API
• Structured error returns ({ success: false, error }) consistent with other integrations — never lets HTTP failures propagate silently

Closes #14

Files Changed

New files (7):

• packages/nodes/src/integrations/hunter/credentials.ts — hunterCredential definition with query-param auth and testRequest pointing to /account
• packages/nodes/src/integrations/hunter/schemas.ts — Zod input/output schemas and inferred types for all 3 operations
• packages/nodes/src/integrations/hunter/hunter-domain-search.ts — domain search node implementation with full email/source camelCase mapping
• packages/nodes/src/integrations/hunter/hunter-email-finder.ts — email finder node implementation
• packages/nodes/src/integrations/hunter/hunter-email-verifier.ts — email verifier node implementation with all 10 fields mapped
• packages/nodes/src/integrations/hunter/index.ts — barrel export for all nodes, schemas, types, and credential
• packages/nodes/src/integrations/hunter/__tests__/hunter.test.ts — 47 tests (credentials, schemas, all 3 nodes, integration flow)

Modified files (3):

• packages/core/src/types/node.ts — added hunter?: { apiKey: string } to the NodeCredentials interface
• packages/nodes/src/integrations/index.ts — re-exports Hunter nodes, schemas, types, and credential from the integrations barrel
• packages/nodes/src/index.ts — adds Hunter exports to the main package index and includes the 3 nodes in the builtInNodes array

Acceptance Criteria

• Credential definition with Zod schema
• All 3 operations implemented
• Zod schemas for inputs/outputs
• Unit tests
• Error handling

Test Plan

• 47 new Hunter tests (credentials, schemas, all 3 nodes, integration flow)
• Happy path for each operation with mocked Hunter.io responses
• Error paths: 401, 429, 5xx, network errors, missing credentials
• URL parameter verification (api_key in query, snake_case params, URL-encoding of special chars)
• snake_case -> camelCase output mapping verified
• Empty results handling
• Integration test: domain search -> email finder -> email verifier flow
• All 278 tests pass (231 baseline + 47 new, zero regressions)
• Typecheck passes for Hunter files