Rust SDK: add typed per-session capability controls

rust/README.md

@@ -78,6 +78,53 @@ client.stop().await?;
 
 With the default `CliProgram::Resolve`, `Client::start()` resolves the CLI in this order: an explicit `CliProgram::Path(path)`, the `COPILOT_CLI_PATH` env var, then the bundled CLI that was embedded at build time. There is no PATH scanning — if you've opted out of bundling (`default-features = false`) you must supply either `CliProgram::Path` or `COPILOT_CLI_PATH`.
 
+### Session capabilities
+
+The `SessionCapability` enum lets callers enable or disable named runtime features on a **per-session** basis. Capabilities are sent to the runtime as part of the `session.create` / `session.resume` JSON-RPC calls via two wire fields:
+
+- `enabledCapabilities` -- capabilities to opt the session into (extends the `SDK_CAPABILITIES` baseline)
+- `disabledCapabilities` -- capabilities to opt the session out of (disable wins on overlap)
+
+> **Experimental.** Per-session capability controls are an experimental
+> wire-protocol surface and may change or be removed in future SDK or CLI
+> releases.
+
+> **Runtime dependency.** Per-session capability controls require a runtime
+> version that supports `enabledCapabilities` and `disabledCapabilities`.
+> On older runtimes the fields are silently ignored.
+
+Use `SessionConfig::with_enable_capability` / `with_disable_capability` (and their plural counterparts):
+
+```rust,ignore
+use github_copilot_sdk::{SessionCapability, SessionConfig};
+
+let session = client.create_session(
+    SessionConfig::default()
+        .with_enable_capability(SessionCapability::Memory),
+).await?;
+```
+
+On resume, use the same builders on `ResumeSessionConfig`:
+
+```rust,ignore
+use github_copilot_sdk::{ResumeSessionConfig, SessionCapability};
+
+let session = client.resume_session(
+    ResumeSessionConfig::new(session_id)
+        .with_enable_capability(SessionCapability::Memory),
+).await?;
+```
+
+**Variants.** See the `SessionCapability` enum's own documentation for the
+authoritative set of capabilities and their wire names.
+
+**Disable-wins semantics.** If the same capability appears in both
+`enabled_capabilities` and `disabled_capabilities`, disable wins. The runtime
+starts from an `SDK_CAPABILITIES` baseline; enabled capabilities extend it and
+disabled capabilities remove from it, in that order. `SessionCapability::Unknown`
+exists only as a generated deserialization fallback and is rejected if supplied
+to the create/resume capability builders.
+
 ### Session
 
 Created via `Client::create_session` or `Client::resume_session`. Owns an internal event loop that dispatches CLI callbacks to the focused handler traits you install on `SessionConfig`, and broadcasts session events through `subscribe()`.
@@ -714,6 +761,11 @@ gets to be Rust here — cross-SDK parity for these is a post-release
 conversation, not a release blocker. None of these are deprecated and
 none of them are scheduled for removal.
 
+- **`SessionCapability` enum** -- typed enum for per-session
+  capability opt-in / opt-out. Sent via `enabledCapabilities` /
+  `disabledCapabilities` on the `session.create` and `session.resume` wire
+  calls. See [Session capabilities](#session-capabilities) above.
+  Experimental.
 - **Typed newtypes** — `SessionId` and `RequestId` are `#[serde(transparent)]`
   newtypes around `String`, so the type system distinguishes a session
   identifier from an arbitrary `String` at compile time. Node/Python/Go