.NET: Hosting.Channels surface - Phase 1

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## Scope

Port the Python `feature/python-hosting` work (the "hosting channels" surface) to .NET. Tracks the staged delivery onto the existing `microsoft/agent-framework:feature/dotnet-hosting-channels` integration branch.

Spec: [`docs/specs/003-dotnet-hosting-channels.md`](https://github.com/microsoft/agent-framework/blob/feature/dotnet-hosting-channels/docs/specs/003-dotnet-hosting-channels.md) (lands with the first PR).

## Why

Today every wire format (Responses, A2A, AG-UI, Functions, Foundry hosting) has its own `Map*` extension and its own state-and-identity story bolted on. As more transports land (Telegram, Discord, Slack, Activity Protocol), the surface area multiplies and cross-channel scenarios (link a Telegram identity to a Responses session, fan a response out to multiple destinations, etc.) have no consistent place to live.

The hosting-channels surface introduces a single composable seam — one `AgentFrameworkHost` per target, many `Channel`s mounted side-by-side — with shared infrastructure for authorization, identity linking, link policies, host-execution state, durable push, and response-target routing. Python locked the design first; this issue tracks the literal .NET port.

## Posture

Translate, not redesign. Python is canonical. Names, taxonomy, wire-level behaviors mirror Python; ASP.NET Core mechanics only where they have to. Net-new in v1; existing `Hosting.OpenAI` / `Hosting.A2A` / `Hosting.AGUI.AspNetCore` / `Hosting.AzureFunctions` / `Foundry.Hosting` packages stay untouched.

## Delivery plan

Staged PRs against `feature/dotnet-hosting-channels`.

1. **Core scaffold** — `Microsoft.Agents.AI.Hosting.Channels` (`Channel`, `ChannelContribution`, capability interfaces, `ChannelRequest`, `ResponseTarget`, `HostedRunResult`, `HostedStreamItem`, `AgentFrameworkHost` + `AuthorizeAsync` pipeline, `IAgentFrameworkHostBuilder`, `Map/AddAgentFrameworkHost` extensions, `IsolationKeys`).
2. **Identity + state + runner** — `IIdentityAllowlist` family + combinators + `AuthorizationProfile`, `IIdentityLinker`, `ILinkPolicy` + 4 built-ins, `IHostStateStore` + `InMemory` and `File` impls, `IDurableTaskRunner` + `InProcessDurableTaskRunner`, `OneTimeCodeIdentityLinker`, `ResponseRouter` (registers `hosting.push`, walks response targets, fans out, runs per-destination hooks, echo idempotency).
3. **Channel: Invocations** — `Microsoft.Agents.AI.Hosting.Channels.Invocations`. JSON `POST /invocations/invoke` + background + polling. `WorkflowRunResult` rendered as `awaiting_input` / `completed` / `failed`.
4. **Workflow runner** — drives `InProcessExecution.RunStreamingAsync`, persists resume tokens on `IHostStateStore`, resumes via `ExternalResponse`.
5. **Channel: Telegram** — `Microsoft.Agents.AI.Hosting.Channels.Telegram`. Polling + webhook over raw HTTP. `IChannelPush`. `ConversationScope` (`PerUser` / `PerUserPerConversation` / `PerConversation`). `AcceptInGroup` (`MentionOnly` default for groups). DM-redirect link challenge. `setMyCommands` at startup.
6. **Sample + unit tests** — one sample per channel under `samples/04-hosting/HostingChannels/`. `Microsoft.Agents.AI.Hosting.Channels.UnitTests` covering response target / allowlists / state store / linker / durable runner.
7. **Fast-follow** (separate issues): `Microsoft.Agents.AI.Hosting.Channels.Responses` (Responses wire-compat), `FoundryHostedAgentRunner` in `Microsoft.Agents.AI.Foundry.Hosting`, `IsolationKeysMiddleware` (ASP.NET Core header lifting), workflow-runner durability across process restarts, per-channel `IChannelPushCodec` (for JSON-payload durable runners), `.DurableTask` / `.Discord` / `.Activity` / `.EntraId` packages.

## Active PRs

- [#6151](https://github.com/microsoft/agent-framework/pull/6151) — draft scaffold (commits 1–6 above)

## Out of scope (v1)

- Rewriting any existing `Map*` extension to delegate to the new builder. Tier-2 migration ships as a separate, fast-follow release once v1 is stable.
- Any hard break in alpha or shipped packages.

## References

- Python source branch: [`feature/python-hosting`](https://github.com/microsoft/agent-framework/tree/feature/python-hosting)
- Python Discord channel PR (fast-follow reference): #6081
- Python hosting spec (the spec this .NET work ports): `docs/specs/002-python-hosting-channels.md` on the Python branch

Start with docs/specs/003-dotnet-hosting-channels.md and compare the Python feature/python-hosting branch, since Python is canonical. Review draft PR #6151 and the staged delivery plan to identify the remaining phase. Done means the listed .NET hosting-channels phases and their specified tests and samples are delivered without changing the existing hosting packages.

Written by the indexing model from the issue text.