feat(sdk): add customer-hosted ChatKit providers

.changeset/custom-chatkit-providers.md

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+---
+"@trytilde/sdk": minor
+"@trytilde/sdk-vercel-ai-node": minor
+"@trytilde/api-client": minor
+---
+
+Support customer-hosted ChatKit providers with signed Fetch handlers, scoped runtime clients, contextual session tools, rich delivery, and Linq/AgentMail examples. Deploy compatible API support before using the new methods.

docs/adrs/0043-custom-chatkit-provider-sdk.md

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+# ADR-0043: Framework-neutral ChatKit provider authoring
+
+Status: Accepted
+
+## In brief
+
+- Export authoring and runtime helpers from a dedicated ChatKit provider subpath.
+- Separate administrative, connection-runtime, and end-user credentials.
+- Keep platform behavior in TypeScript and canonical accounting in Tilde.
+
+## Context
+
+A custom provider must express setup, ingestion, rich sends, identities, contextual
+session tools, and recovery without requiring a specific agent framework or a
+separately provisioned toolkit for each action.
+
+This extends [SDK ownership](0030-tilde-sdk-and-cli-ownership.md) and preserves
+[org/runtime identity separation](0042-org-runtime-identities.md). Platform
+extensions follow [ADR-0038](0038-metadata-is-extension-data.md).
+
+## Decision
+
+Use standard Fetch Request/Response handlers for signed discovery and versioned
+operations. Validate declared schemas/capabilities and keep server-bound execution
+context outside model arguments. Provide connection-scoped ingestion, attachment,
+conversation, and participant methods. Expose administrative wrappers separately.
+
+Use stable execution/delivery IDs and explicit applied/absent/uncertain
+reconciliation. Private rendering and recipient options flow through canonical
+send preparation; reference email adapters redact BCC from public outputs.
+Streaming protocol adapters submit canonical turns and relay cursor-based events
+using the requesting user's authenticated client. Vercel AI adaptation is optional.
+
+## Consequences
+
+The core provider entry point has no Vercel AI requirement. Linq and AgentMail
+serve as executable reference adapters, while hosts own deployment, secret
+storage, webhook verification, and platform recovery. Backend support must ship
+before an SDK release exposes these methods to customers. The example Node host
+uses durable encrypted SQLite locally; Tilde's canonical database remains Postgres.

docs/updates/154.md

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+# Customer-hosted ChatKit SDK
+
+PR: https://github.com/trytilde/dispatch/pull/154 (draft)
+
+## Intent of the change
+
+Bring the custom ChatKit provider implementation into Dispatch's supported SDK
+packages so customers can host complete transports and contextual tools using
+`@trytilde/sdk`, with no dependency on the archived Harness SDK repository.
+
+## Architecture changes
+
+See [ADR-0043](../adrs/0043-custom-chatkit-provider-sdk.md), extending ADR-0030,
+ADR-0038, and ADR-0042. Tilde owns authorization and canonical durable execution;
+customer backends own platform behavior. SDK, connection-runtime, and user-client
+credentials remain separate.
+
+```mermaid
+flowchart LR
+  API[Tilde API] -->|signed operations| Provider[Customer TypeScript provider]
+  Provider --> Platform[Platform API]
+  Provider -->|scoped normalized events| API
+  Caller -->|user credentials| Stream[Canonical session operations and SSE]
+```
+
+## Summarized changes
+
+- Port provider authoring/runtime, definition and connection management, rich
+  delivery, work diagnostics, and canonical session streams to `packages/sdk`.
+- Integrate dynamically discovered custom tools into the existing Vercel AI
+  `context.session.tools` object and export `sessionProviderTools`.
+- Preserve newer room, run, identity, and client APIs. Add the Linq/AgentMail
+  reference implementations and streaming example under the owning SDK package.
+- Regenerate contracts from API PR https://github.com/trytilde/api/pull/276.
+  Companion public docs: https://github.com/trytilde/docs/pull/36.
+- Metadata classification: provider-specific external thread/message details are
+  interpreted only by the Linq or AgentMail reference adapter. The SDK forwards
+  opaque extension objects; canonical routing, credentials, leases, and execution
+  IDs use the signed typed contract, not metadata.
+- No OpenBot client surface or fork-owned configuration is changed. No new
+  external service or global tool install is required; the existing Node 24
+  toolchain is used and Ajv reuses the repository's current dependency version.
+- Validation on Dispatch `fcee72c`: `openbot sdk refresh` regenerated the client,
+  validated 732 operations, built all SDK packages, and passed all 316 SDK tests against the final rebased API contract:
+  core SDK 150, Vercel Node 152, and other SDK packages 14. `openbot sdk validate`
+  passed package/export validation; the final changed SDK packages were rebuilt.
+- Focused typechecking, example typechecking, formatting, and lint pass. Existing
+  no-base-to-string warnings remain in older portions of the webhook test file.
+  Full OpenBot application check/build and browser/Electron e2e were not run
+  because this changes server SDK packages rather than application surfaces.
+
+## Critical to apply
+
+yes
+
+Deploy the additive API and upgrade workers before releasing the SDK or enabling
+custom providers. Consumers use `@trytilde/sdk/chatkit-provider` and configure
+backend signing keys, per-connection runtime tokens, and durable backend state.
+The reference host's setup guide covers the manually provisioned AgentMail
+webhook. No publication, deployment, or live-platform/browser e2e is included.
+
+Record refreshed after final wallet-preserving contract refresh: 2026-09-07T19:51:48+00:00.