feat: CLI syntax highlighting, hints, and subcommand completion

[kacy]

summary

makes the CLI feel polished with three improvements:

• syntax highlighting — known commands in bold cyan, unknown in red, quoted strings in green. re-highlights on every keystroke for real-time feedback.
• inline hints — shows argument synopsis (dimmed) after typing a command and a space. for multi-word commands (CLUSTER, SLOWLOG, PUBSUB), shows available subcommands as a hint.
• subcommand completion — tab-completes the second token for CLUSTER (14 subcommands), SLOWLOG (3), and PUBSUB (3). also completes local commands (help, quit, exit, clear).

what was tested

• 47 CLI tests pass (up from 32), including:
  • 5 highlighter tests (known/unknown/local commands, empty line, quoted strings)
  • 4 hinter tests (args for known commands, subcommands for CLUSTER, unknown command, cursor position)
  • 6 subcommand metadata tests (cluster/slowlog/pubsub content, sorting, unknown parent)
• cargo clippy --workspace -- -D warnings clean
• manual verification: typed commands show correct colors, hints appear, tab completion works for subcommands