Add SSH cert flows and user service installer

docs/adr/0015-user-service-installation.md

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+# ADR 0015: User Service Installation
+
+## Status
+
+Accepted.
+
+## Decision
+
+Geth provides `geth daemon service ...` commands to install, uninstall, start,
+stop, inspect, and print daemon service definitions.
+
+The service is always installed as a user service:
+
+- Linux: systemd user unit, `~/.config/systemd/user/geth.service`
+- macOS: launchd user agent, `~/Library/LaunchAgents/local.geth.daemon.plist`
+- Windows: per-user scheduled task named `geth-daemon`
+
+The service runs the same single binary as `geth daemon run` and preserves
+`GETH_HOME` in the service environment.
+
+## Consequences
+
+There are still no separate `gethd` or `gethctl` binaries. Installation does not
+require privileged system service access. Linux users who need the daemon before
+interactive login may still need platform-specific user-service setup such as
+systemd lingering.
+
+Future service managers can be added behind the same service-manager enum and
+CLI shape.