docs: add security policy and update readme - #25

Summary

• add SECURITY.md with vulnerability disclosure process
• update README.md to reflect all current features and commands

SECURITY.md

• vulnerability reporting guidelines (private disclosure)
• supported versions table
• security considerations:
  • network exposure (default localhost)
  • memory limits for production
  • untrusted input handling
  • persistence file permissions

README.md updates

• complete command list with newly added commands:
  • MGET, MSET, INCR, DECR
  • SCAN, FLUSHDB
  • PERSIST, PTTL, PEXPIRE, ZCARD
• configuration reference table with all CLI flags
• persistence example with --appendonly
• security section linking to SECURITY.md
• note about authentication not yet implemented
• link to CONTRIBUTING.md
• mark ember-cluster as WIP

Test plan

• no code changes, documentation only