fix: honor SSH connection settings consistently

[krunkosaurus]

Remote collector commands currently use different SSH options from their readiness configuration. As a result, SSH_CONTROL_PERSIST_SECONDS=0 still enables a 300-second master, and changes to the configured key or password do not change the socket passed to SSH.

Use the existing private control-socket configuration for both readiness and command execution. The documented persistence value and its bounds now reach SSH, zero disables both creating and joining a master, and the existing global switch and independent tunnel behavior remain supported. Retain SSH_CONTROL_PERSIST as a numeric fallback when the documented setting is unset. Add a host-memory troubleshooting note linking the separate upstream polkit fix.

Follow-up to #84 and #85; related to #73.

Validation:

• Expanded SSH suite: 18/18 pass; eight cases failed against unchanged main.
• npm test: 325 server tests and 23 frontend tests pass.
• npm run typecheck, npm run build, and git diff --check pass.
• Live read-only check using Node 22 and the Docker OpenSSH client against a DGX Spark: three concurrent reads plus a later read shared one Linux login session; master check passed, socket path was 50 bytes, and idle persistence was 60 seconds. The test closed its own master afterward.