feat(cli): launch file observer

[khaliqgant]

Summary

• add relayfile observer [WORKSPACE] to open the hosted file observer for the resolved workspace
• default the hosted observer to https://agentrelay.com/observer/file, matching the live AgentRelay router path
• pass server, token, and workspace to the observer through a URL fragment so the token is not sent to the observer host as a query parameter
• teach the file-observer app to read runtime config from URL fragment/search params
• document RELAYFILE_OBSERVER_URL, ignore generated observer output, and remove stale files.relayfile.dev route references

Tests

• go test ./cmd/relayfile-cli
• go test ./...
• npm test --prefix packages/file-observer
• npm run build --prefix packages/file-observer

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