Hoist _meta onto CustomizationBase so every customization type carries provider metadata uniformly

[colbylwilliams]

Summary

_meta previously existed on exactly one customization type — AgentCustomization — leaving every other customization (plugin, directory, skill, prompt, rule, hook, MCP server) with no provider-specific escape hatch. This hoists the optional _meta slot from AgentCustomization up to the shared CustomizationBase, so every customization type carries one uniformly, and drops the now-redundant per-type declaration on AgentCustomization.

Fixes #316.

Independent validation

I verified the issue's premises against the current tree rather than taking them on faith:

• Current state matches the issue. CustomizationBase (types/channels-session/state.ts) had id/uri/name/icons?/range? and no _meta; AgentCustomization was the only type with a _meta slot. Confirmed via git history that _meta was added to AgentCustomization as a deliberate one-off (e3afb2d), not as a considered "agents only" design decision.
• Aligns with doctrine. docs/guide/doctrine.md explicitly endorses provider-specific metadata as an escape hatch ("AHP keeps escape hatches explicit") and lists "Is provider-specific metadata an enhancement rather than a requirement?" as a passing design test. _meta is optional and opaque, so a minimal client can ignore it and still render a coherent session — it does not smuggle agent implementation details across the host boundary. No contradiction with foundational principles.
• Wire-compatible. The key, shape, and semantics of _meta are unchanged; AgentCustomization still exposes _meta (now by inheritance), verified in all four statically-typed client mirrors. Every other type simply gains the optional slot.

Changes

• types/channels-session/state.ts: add _meta? to CustomizationBase, remove it from AgentCustomization.
• Regenerated JSON schemas + all client mirrors (Rust / Kotlin / Swift / TypeScript / Go). Verified in state.schema.json that all eight concrete customization types gained _meta (false → true) while AgentCustomization retained it, and that AgentCustomization still carries the field in every generated client.
• docs/guide/customizations.md: new "Provider metadata" note describing _meta as the uniform, opaque escape hatch (prefer a first-class field where data has a natural home).
• CHANGELOG entries in the root and all five client changelogs.

Verification

• npm run generate + npm test — all tests pass (typecheck, lint, release-metadata, changelog, reducer coverage, schema self-consistency).
• Go go build ./... + gofmt -l clean.
• Rust cargo fmt --check + cargo clippy clean.
• Swift swift build clean.
• Kotlin verified by inspection (generated code matches the established, already-compiling @SerialName("_meta") pattern; no local JDK/podman to build).

Relationship to #315 / #317

Complementary. This is the generic fallback for provider-specific data with no first-class home; #317 (native version on PluginCustomization, closing #315) adds a typed field where the concept has a natural home. main has been merged into this branch, so both changes coexist: PluginCustomization carries both version and _meta, and every other customization type gains _meta.

[Copilot]

Pull request overview

This PR updates the AHP session customization model to support a uniform provider-specific metadata escape hatch across all customization variants by hoisting the optional _meta field from AgentCustomization to the shared CustomizationBase. It then propagates the change through regenerated JSON Schemas and all client mirrors, and documents the new field’s intended usage.

Changes:

• Added optional _meta?: Record<string, unknown> to CustomizationBase and removed the now-redundant AgentCustomization._meta declaration (inherited now).
• Regenerated schema/*.schema.json artifacts and all language client mirrors so every customization type exposes the field (wire name _meta).
• Updated documentation and changelogs to describe the uniform provider metadata mechanism.

Show a summary per file

File	Description
types/channels-session/state.ts	Hoists _meta onto CustomizationBase and removes the per-AgentCustomization definition.
schema/state.schema.json	Regenerated schema reflecting _meta on customization types; includes generator output fixes.
schema/notifications.schema.json	Regenerated schema reflecting _meta on customization shapes in notification payloads.
schema/errors.schema.json	Regenerated schema reflecting _meta on customization shapes in error payloads; includes generator output fixes.
schema/commands.schema.json	Regenerated schema reflecting _meta and fixes prior dangling $ref output.
schema/actions.schema.json	Regenerated schema reflecting _meta and fixes prior dangling $ref output.
docs/guide/customizations.md	Adds guidance for _meta as an opaque, provider-specific escape hatch.
CHANGELOG.md	Adds spec changelog entry for _meta being available on all customization types.
clients/typescript/CHANGELOG.md	Notes the new _meta field availability across customization types in the TS client.
clients/swift/CHANGELOG.md	Notes the new meta (wire _meta) field availability across customization types in the Swift client.
clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/State.generated.swift	Regenerated Swift types to include meta (wire _meta) on customization structs.
clients/rust/crates/ahp-types/src/state.rs	Regenerated Rust types to include meta (wire _meta) on customization structs.
clients/rust/CHANGELOG.md	Notes the new meta (wire _meta) field availability across customization types in the Rust client.
clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/State.generated.kt	Regenerated Kotlin types to include meta (wire _meta) on customization data classes.
clients/kotlin/CHANGELOG.md	Notes the new meta (wire _meta) field availability across customization types in the Kotlin client.
clients/go/CHANGELOG.md	Notes the new Meta (wire _meta) field availability across customization types in the Go client.
clients/go/ahptypes/state.generated.go	Regenerated Go types to include Meta (wire _meta) on customization structs.

Review details

• Files reviewed: 15/17 changed files
• Comments generated: 0
• Review effort level: Low