Add session.handoff :host field spec and docs (from PR #65)

Port upstream PR #900 change: session.handoff event data now
includes optional :host and :remote-session-id fields. Adds
::session.handoff-data and ::remote-session-id specs. Updates
API.md event table with data shape documentation.

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

Author: krukow

CHANGELOG.md

@@ -8,6 +8,7 @@ All notable changes to this project will be documented in this file. This change
 - **UI Elicitation convenience API** — new public functions `confirm!`, `select!`, `input!` wrap the existing `ui-elicitation!` with typed schemas. `capabilities` accessor returns host capabilities from session create/resume response. `elicitation-supported?` predicate checks if the host supports elicitation dialogs. All convenience methods throw with a clear error when elicitation is unsupported (upstream PR #906).
 - **`COPILOT_CLI_PATH` env var fallback** — client constructor now checks `COPILOT_CLI_PATH` environment variable before defaulting to `"copilot"` when no explicit `:cli-path` or `:cli-url` is provided (upstream PR #906).
 - **New event type** `session.custom_agents_updated` added to event type enum.
+- **`:host` field on `session.handoff` events** — event data now includes optional `:host` field with the GitHub host URL. New `::session.handoff-data` spec documents the shape (upstream PR #900).
 - New specs: `::command-definition`, `::commands`, `::session-capabilities`, `::elicitation-params`, `::elicitation-result`, `::input-options`.
 - Function specs and instrumentation for `capabilities`, `elicitation-supported?`, `confirm!`, `select!`, `input!`.
 - Integration tests for command wire format, command.execute routing, unknown command errors, handler errors, capabilities storage, and elicitation guards.

doc/reference/API.md

@@ -1093,7 +1093,7 @@ Convert an unqualified event keyword to a namespace-qualified `:copilot/` keywor
 | `:copilot/session.idle` | Session finished processing |
 | `:copilot/session.info` | Informational session update |
 | `:copilot/session.model_change` | Session model changed |
-| `:copilot/session.handoff` | Session handed off to another agent |
+| `:copilot/session.handoff` | Session handed off to another agent; data: `{:remote-session-id "..." :host "https://github.com"}` (both optional) |
 | `:copilot/session.usage_info` | Token usage information |
 | `:copilot/session.context_changed` | Session context (cwd, repo, branch) changed |
 | `:copilot/session.title_changed` | Session title updated |

src/github/copilot_sdk/specs.clj

@@ -580,6 +580,11 @@
 
 (s/def ::session.idle-data map?)
 
+(s/def ::remote-session-id string?)
+
+(s/def ::session.handoff-data
+  (s/keys :opt-un [::remote-session-id ::host]))
+
 (s/def ::agent-mode #{:interactive :plan :autopilot :shell})
 (s/def ::interaction-id string?)