docs(book): add node operator guide

[Sahilvishnaliya]

Summary

Adds a new top-level book chapter: the Node Operator Guide (explain/operator-guide.md) — a field manual for people who run Psyche training nodes over time, complementing the existing one-time setup docs (Quickstart: Compute Provider, Joining a run).

What it covers:

• What you actually operate: the split between run-manager (supervisor), the auto-pulled client container, the Solana keypair, RPC endpoints, and the host GPU stack — and which parts are managed for you vs. manual.
• Node lifecycle from the operator's seat: a mermaid diagram of the epoch phase machine (WaitingForMembers → Warmup → RoundTrain/RoundWitness → Cooldown), a per-phase table of what your node is doing and the typical operator-side failure in that phase, and which log events mark epoch boundaries.
• Rewards accrue per completed epoch: points are shared equally among clients that finish the epoch; leaving mid-epoch forfeits them.
• Running long-term: tmux/systemd session management, a startup checklist (GPU-in-Docker check, wallet funding for tx fees, disk), and what to monitor (container, phase transitions, GPU utilization, RPC health).
• Checking run state from the CLI: the read-only run-manager list-runs, json-dump-run, and json-dump-user commands for answering "is it my node or the run?".
• Updating: which upgrades are automatic vs. manual, and what version-mismatch loops mean.
• Maintenance windows: why right after an epoch ends is the safe moment to stop.
• Delegation and multi-machine setups: one keypair per machine, delegate keypairs via join-authorization-delegate, master-key reward identity for cloud-burst fleets.
• Claiming rewards: run-manager treasurer-claim-rewards usage.
• Troubleshooting quick reference: a symptom → likely-cause table linking back to the Client FAQ.

Notes for reviewers

• Technical claims verified against the codebase: run state machine in shared/coordinator/src/coordinator.rs (RunState, CoordinatorConfig), epoch-end reward accounting in the Solana coordinator program (instance_state.rs: healthy finishers earn earning_rate_total_shared / num_finishers), and the run-manager CLI surface in tools/rust-tools/run-manager/src/main.rs (list-runs, json-dump-run, json-dump-user, treasurer-claim-rewards, join-authorization-*).
• Purely additive: one new page + one SUMMARY.md line.
• Happy to trim sections, adjust tone, or move the chapter elsewhere in the book if you'd prefer it nested under an existing section.