Add usage and billing metrics docs page (#1720)

* Add usage and billing metrics docs page

Document how SDK integrators read token counts, context-window
utilization, AI credit cost, and account quota via session events and
RPC methods (assistant.usage, session.usage_info,
session.metadata.contextInfo, session.usage.getMetrics, models.list,
account.getQuota).

Examples are provided for TypeScript, Python, Go, .NET, Java, and Rust,
and the compiled snippets pass the docs-validation harness. Adds a link
to the features index.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* docs: fix assistant.usage cost field in Go/Java/Rust examples; clarify contextInfo token limit semantics

- Add cost field (with nil/Option guard) to Go, Java, and Rust assistant.usage
  snippets so all six languages match the documented field table and the
  'functionally equivalent' claim in the intro
- Correct the description of session.metadata.contextInfo parameters:
  promptTokenLimit passes 0 to use the runtime default, while
  outputTokenLimit passes 0 when the value is unknown (per generated types)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Steve Sanderson <SteveSandersonMS@users.noreply.github.com>

Author: 3 people

docs/features/README.md

@@ -18,6 +18,7 @@ These guides cover the capabilities you can add to your Copilot SDK application.
 | [Session limits](./session-limits.md) | Set an AI Credits budget for a session and observe budget events |
 | [Image Input](./image-input.md) | Send images to sessions as attachments |
 | [Streaming Events](./streaming-events.md) | Subscribe to real-time session events (40+ event types) |
+| [Usage and Billing](./usage-and-billing.md) | Read token counts, context-window utilization, AI credit cost, and account quota |
 | [Steering & Queueing](./steering-and-queueing.md) | Control message delivery—immediate steering vs. sequential queueing |
 | [Session Persistence](./session-persistence.md) | Resume sessions across restarts, manage session storage |
 | [Remote Sessions](./remote-sessions.md) | Share locally hosted sessions to GitHub web and mobile via Mission Control |