feat(run-manager): add keygen command to generate a new Solana keypair

[Sahilvishnaliya]

Summary

Adds a run-manager keygen command that generates a new Solana keypair and writes it to disk in the same JSON format the solana-cli produces, so creating a wallet no longer requires installing the solana binaries.

Closes #500.

Usage

run-manager keygen --output ~/.config/solana/psyche-node.json
# Wrote new keypair to /home/user/.config/solana/psyche-node.json
# Public key: 7xKXtg2CW87d97TXJSDpbD5jBkheTqA83TZRuJosgAsU

• The output file is the standard 64-byte JSON array (identical to solana-keygen new --outfile), directly usable as WALLET_PATH / WALLET_PRIVATE_KEY_PATH in the .env file.
• Refuses to overwrite an existing file unless --force is passed.
• write_to_file creates parent directories and restricts permissions to 0600 on unix (both provided by solana_sdk's EncodableKey implementation).
• Purely offline — no cluster connection, so it's usable without an .env or RPC endpoint.

Notes for reviewers

• Follows the existing command conventions: #[derive(Debug, Clone, Args)] + #[command()], registered in Commands in main.rs; dispatched without a SolanaBackend since it needs no network access.
• Keypair::new() (OS entropy) + EncodableKey::write_to_file are the same primitives solana-keygen uses, via the already-required anchor-client re-export of solana_sdk.
• The book's Joining a run page is updated in a second commit to show run-manager keygen as the primary path (solana-cli kept as an alternative).
• Local cargo check is blocked on Windows by the vendored openssl-sys build (needs perl; CI builds run on Ubuntu with libssl-dev), so compile verification is left to CI — the touched APIs (Keypair::new, write_to_file, pubkey) are verified against solana-sdk 2.1.4 sources.