Add _meta to UserMessage

[colbylwilliams]

Adds an optional _meta?: Record<string, unknown> field to UserMessage so user messages can carry provider-specific metadata, mirroring the MCP _meta convention already used on MessageAttachmentBase, ToolDefinition, ToolCallBase, UsageInfo, and SessionState.

Changes

• types/channels-session/state.ts: add UserMessage._meta with JSDoc describing the MCP-aligned semantics.
• Generated clients: regenerated via npm run generate. _meta lands as the idiomatic optional map type in each client:
  • Go: Meta map[string]json.RawMessage \json:"_meta,omitempty"``
  • Kotlin: @SerialName("_meta") val meta: Map<String, JsonElement>? = null
  • Rust: #[serde(rename = "_meta", default, skip_serializing_if = "Option::is_none")] pub meta: Option<JsonObject>
  • Swift: public var meta: [String: AnyCodable]? with CodingKey meta = "_meta" (and init gains a trailing meta: parameter defaulting to nil)
• Schemas: schema/*.schema.json regenerated; UserMessage picks up an optional _meta object.
• Docs: docs/guide/state-model.md shows _meta in the UserMessage snippet and adds a paragraph distinguishing it from the existing attachment-level _meta blob.
• CHANGELOGs: root spec changelog plus every client changelog (Go, Kotlin, Rust, Swift, TypeScript) gain an Added bullet under ## [Unreleased] per the AGENTS.md "types/** ripples to every client" rule.

Validation

• npm run generate — clean
• npm run typecheck — clean
• npm run lint — clean
• npm run verify:release-metadata — passes
• npm run verify:changelog — passes
• npx tsx --test types/*.test.ts types/version/*.test.ts — 224/224 passing (the npm run test wrapper otherwise trips the pre-existing c8/yargs incompatibility on Node 26 in this dev env, unrelated to this change)

Compatibility

Purely additive. Existing producers and consumers that ignore _meta continue to round-trip messages unchanged, so no protocol version bump is required.

Coordination with #155

#155 renames UserMessage → Message and adds a MessageKind discriminant. This PR was deliberately scoped to a single additive field on UserMessage so the new _meta field will carry forward cleanly into Message._meta when #155 lands (or rebases on top of this).

(Written by Copilot)

[Copilot]

Pull request overview

Adds an optional _meta field to UserMessage so user messages can carry provider-specific metadata, mirroring the existing MCP _meta convention used elsewhere in the protocol (attachments, tool definitions, tool calls, usage info, session state). The change is purely additive and regenerates all client bindings and schemas.

Changes:

• Add _meta?: Record<string, unknown> to UserMessage in types/channels-session/state.ts with JSDoc.
• Regenerate JSON schemas and all client bindings (Go, Kotlin, Rust, Swift) plus update Rust reducer test fixtures.
• Update docs and root + per-client CHANGELOGs under [Unreleased].

Show a summary per file

File	Description
types/channels-session/state.ts	Source-of-truth: adds _meta to UserMessage.
schema/{state,notifications,errors,commands,actions}.schema.json	Regenerated schemas pick up optional _meta on UserMessage.
docs/guide/state-model.md	Documents _meta on UserMessage and contrasts with attachment _meta.
clients/typescript/CHANGELOG.md	Adds [Unreleased] entry.
clients/swift/AgentHostProtocol/.../State.generated.swift	Adds meta property, CodingKey, and init parameter.
clients/swift/CHANGELOG.md	Adds [Unreleased] entry.
clients/rust/crates/ahp-types/src/state.rs	Adds meta: Option<JsonObject> with #[serde(rename = "_meta", …)].
clients/rust/crates/ahp/src/reducers.rs	Updates UserMessage literals in tests with meta: None.
clients/rust/CHANGELOG.md	Adds [Unreleased] entry.
clients/kotlin/.../State.generated.kt	Adds meta field with @SerialName("_meta").
clients/kotlin/CHANGELOG.md	Adds [Unreleased] entry.
clients/go/ahptypes/state.generated.go	Adds Meta map[string]json.RawMessage field.
clients/go/CHANGELOG.md	Adds [Unreleased] entry.
CHANGELOG.md	Adds root spec [Unreleased] entry.

Copilot's findings

• Files reviewed: 16/18 changed files
• Comments generated: 0