feat: add exec and trace CLI commands

[zharinov]

Summary

Add working exec and trace CLI commands that use the runtime engine.

Commands

exec - Execute a query and output JSON:

plotnik exec query.ptk source.ts
plotnik exec -q '(identifier) @id' -l javascript source.ts

trace - Execute with step-by-step trace output:

plotnik trace query.ptk source.ts

Changes

• exec.rs - Now functional, executes queries and outputs JSON
• trace.rs - New command for debugging execution
• run_common.rs - Shared utilities for both commands
• cli.rs - Added trace command, updated exec arguments
• main.rs - Wire up both commands

Test plan

• cargo check passes
• Clippy passes