Add set operations: SINTER / SUNION / SDIFF / SPOP / SRANDMEMBER #9

Summary

Add set operation commands: SINTER, SUNION, SDIFF, SPOP, and SRANDMEMBER.

Why

RAMen has the basic set commands (SADD, SREM, SMEMBERS, SISMEMBER, SCARD) but none of the multi-set operations or random/pop helpers, which are core to using sets as a real data type.

Where

• Handlers: internal/server/cmd_set.go
• Register in internal/server/commands.go (the sets block)
• Storage: internal/store/set.go

Acceptance criteria

• Multi-key operations handle missing keys as empty sets
• SPOP and SRANDMEMBER support the optional count argument
• Tests added covering multi-set edge cases
• Documented in docs/commands.md