fix(browser): make observation start replay idempotent

[kinzhi]

Core retransmits the one-way browser.observer.command start while it waits for the started lifecycle event. The Worker treated each retransmission as a new observer, so the recovery path Core added turned into a self-conflict.

The Worker now records the command_id alongside the lease and returns early when both match the observation already opening, so a delivery replay does not replace the stream or report that the same observer conflicts with itself.

This is also a release blocker: the root repository's main already pins this commit as its openlinker-cli submodule pointer, and the commit was never pushed, so a recursive clone or a CI submodule checkout from main currently fails on a SHA that does not exist on the remote.

TestObservationStartReplayIsIdempotent covers the replay; go test ./pkg/agentexec/ ./pkg/browserprotocol/ ./pkg/browserruntime/ passes.

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