G5: wait for Chromium network idle

[SarthakWade]

Parent: #54, roadmap backlog G5.

Goal

Add a bounded network-idle predicate to the existing wait command so agents can wait for Chromium page traffic without polling diagnostics or guessing delays.

Contract

• Add wait --network-idle and a boolean networkIdle protocol parameter.
• Combine URL, text, settled, and network-idle predicates with logical AND.
• Preserve current bare wait behavior: it still defaults to --settled.
• wait --network-idle alone requests only network idle unless --settled is also present.
• Chromium reports idle only after zero qualifying in-flight requests for a fixed 500 ms quiet window measured from command start or the latest qualifying network event.
• Track requests from Network.requestWillBeSent through Network.loadingFinished or Network.loadingFailed, including redirects without double-counting request IDs.
• Do not let persistent WebSocket or EventSource connections block forever. Ordinary fetch/XHR long polling remains qualifying and may time out.
• Keep the existing 100 through 120000 ms timeout bound and return the existing typed timeout error.
• WebKit must fail immediately with UNSUPPORTED_CAPABILITY when network idle is requested. It must not approximate this through the page-world diagnostics bridge.
• Capabilities must declare network-idle wait support separately for each engine.
• Return the existing bounded page-state result. Do not expose request URLs, headers, bodies, or identifiers.

Required implementation surfaces

• Architecture decision.
• CLI parser and help.
• Protocol validation and generated schema.
• Shared engine contract and capability matrix.
• Chromium CDP event tracking.
• WebKit explicit capability failure.
• Command reference, phase contract, skill guidance, and backlog entry.
• Generated SDK schema and clients where required by the repository contract.

Acceptance criteria

• Parser and validation tests cover the flag, conjunction behavior, bare-wait compatibility, invalid values, and timeout bounds.
• Deterministic tests cover request start, finish, failure, redirect reuse, persistent-connection exclusion, quiet-window reset, and timeout behavior.
• Linux E2E proves a delayed page request blocks until the quiet window and then succeeds.
• macOS E2E proves the predicate fails with UNSUPPORTED_CAPABILITY without waiting for the timeout.
• Capability and schema drift tests pass.
• Existing settled, URL, and text waits remain unchanged.
• No network data is added to command output, logs, flows, or artifacts.