fix(core): route relay through multi-workspace bindings

[xxb]

Summary

This fixes bot-to-bot relay in multi-workspace mode.

Before this change, relay requests only passed chatID into the target engine, so the target side bypassed normal workspace routing and used the global agent/session manager. That caused relayed messages to run in the wrong workspace.

What changed

• pass the full source sessionKey through relay handling
• resolve relay workspace on the target side from the source session context
• route relay turns to the workspace-specific agent and session manager in multi-workspace mode
• use a platform-qualified relay session key to avoid cross-platform collisions
• keep CC_SESSION_KEY set to the original source session key for agent-side relay/cron context
• add regression tests for:
  • single-workspace relay staying on the global agent
  • multi-workspace relay routing by source session key
  • multi-workspace relay failing cleanly when no workspace binding exists
  • existing relay timeout behavior

Behavior note

In multi-workspace mode, relay from an unbound source channel now fails explicitly instead of silently falling back to the global agent. This is intentional so relay behavior matches workspace-aware routing expectations.

Validation

nix shell nixpkgs#go -c go test ./core -run 'TestHandleRelay|TestSessionContextForKey|TestMultiWorkspaceResolution' -count=1
nix shell nixpkgs#go -c go test ./core -count=1
nix shell nixpkgs#go -c go test ./... -run '^$' -count=1

[chenhg5]

LGTM. Well-implemented multi-workspace relay routing.

Review notes:

• ✅ CI passes
• ✅ Correct approach: routes relay through workspace bindings
• ✅ Uses sourceSessionKey to resolve workspace properly
• ✅ Falls back to global agent/sessions when not multi-workspace
• ✅ Tests cover both single and multi-workspace scenarios
• ✅ Error handling for missing workspace bindings

This enables relay to work correctly with workspace isolation. Approved for merge.

[xxb]

Thanks for the review.

This PR is ready to merge as-is:

• fixes relay routing to the correct workspace in multi-workspace mode
• preserves existing single-workspace behavior
• adds regression coverage for both paths

It is also still cleanly mergeable right now; merging sooner may help avoid future drift/conflicts since core/engine.go is seeing active changes on main.