feat: multi-workspace runtime support

[khaliqgant]

Implements multi-workspace support for the relay runtime, allowing one agent process to connect to multiple Relaycast workspaces simultaneously.

Changes

Auth & Session Model (~565 lines)

• auth.rs: CredentialCache → CredentialSet with memberships[] and default_workspace_id
• New multi_workspace.rs: MultiWorkspaceSession managing N WebSocket connections with merged event fan-in
• Legacy RELAY_API_KEY auto-upgrades to single-entry membership set
• New RELAY_WORKSPACES_JSON env var for multi-workspace bootstrap

Event Routing (~100 lines)

• types.rs: workspace_id and workspace_alias added to InboundRelayEvent, BrokerCommandEvent, InjectRequest
• message_bridge.rs: accepts workspace context, emits workspace-scoped events
• routing.rs: routes by (workspace_id, target) instead of just target

Runtime Flows (~700 lines)

• main.rs: merged receive loop, workspace-aware self-echo filtering, history/replay
• listen_api.rs: /api/send accepts workspaceId/workspaceAlias, /api/config reports memberships
• wrap.rs: workspace-qualified PTY injection when multi-workspace active
• spawner.rs: passes RELAY_WORKSPACES_JSON and RELAY_DEFAULT_WORKSPACE to child processes

Backwards Compatibility

• Legacy single-workspace env vars still work
• Old cache files auto-promote to v2 CredentialSet
• /api/send without workspace valid when single membership or default set

Compiles clean (cargo check passes).

See SPEC.md on feature/multi-workspace-support branch for full architecture details.

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 6 additional findings in Devin Review.

[devin-ai-integration]

🔴 Retry injection writes newline+spaces instead of carriage return to PTY

In the retry injection path, the old code pty.write_all(b"\r") (carriage return / Enter key to submit the injected text) was accidentally replaced with a raw multi-line byte string b"\n " (a newline followed by 8 spaces). This means the re-injected message will never be submitted to the CLI — the PTY receives garbage whitespace instead of an Enter keystroke. The original code at src/wrap.rs before this PR used let _ = pty.write_all(b"\r"); which correctly pressed Enter. The non-retry injection path at src/wrap.rs:916 still correctly uses pty.write_all(b"\r"), confirming this was a reformatting accident during the multi-workspace refactor.

Suggested change

	let _ = pty.write_all(b"
	");
	let _ = pty.write_all(b"\r");

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