feat(cli): add positional shifting for exec/trace commands

[zharinov]

Summary

• When -q is used with a single positional arg, shift it from query to source
• Enables ergonomic usage: plotnik exec -q 'Q = ...' app.js instead of requiring both positional args
• Adds override_usage to show clearer help text for each command variant

Test plan

• CLI tests pass (cargo test -p plotnik-cli)
• Manual testing: plotnik exec -q 'Q = (identifier) @id' app.js