feat: v1.4.0 — 100% documented endpoint coverage, live-verified

[billm950]

Added:

• TagResource ($client->tags()) — workspace tag CRUD (+force delete)
• FieldResource ($client->fields()) — field-definition CRUD, validate, validate-multiple, GET /field-types
• SignerDocumentResource ($client->signerDocuments()) — signer-facing current/list/sign-multiple/decline-multiple/download
• DocumentResource document tags: listTags/appendTags/replaceTags/detachTag
• AssignmentResource: whatsappNotifications(), sequential-signing step pass-through, NOTIFICATION_* constants
• SignerSessionResource: currentDocument (GET /sign), sign, decline
• WebhookResource: eventTypes, dispatches (paginated/filtered), retryDispatch + all 15 subscribable event-type constants
• HttpClientInterface::delete() query support (tag ?force)

Changed:

• WebhookResource::deactivate() now uses the dedicated PUT /accounts/{id}/webhooks/inactivate endpoint
• DocumentResource::assertArtifact() promoted to public static (shared with SignerDocumentResource::download(), DRY)

Tests/docs:

• 116 unit tests (was 77), PHPStan level 8 clean, PSR-12 clean
• 17 live integration tests (4 new: tags, fields, doc-tags, webhook discovery); fixed webhook round-trip to self-clean when no prior config
• README/EXAMPLES/ARCHITECTURE/CHANGELOG updated; fixed stale webhooks()->delete() example; version 1.3.0 -> 1.4.0