feat: unify tree and query AST into ast command

[zharinov]

Summary

• Rename tree command to ast for showing AST of both query and source files
• Show Plotnik query AST when query input is provided (.ptk files or -q)
• Show tree-sitter source AST when source input is provided
• Show both (with headers) when both query and source are provided
• Add unified flags: all commands accept --raw (hidden where not applicable)

Why

The tree command only showed tree-sitter AST of source files. When debugging queries, you often need to see the query's own AST structure. This unifies both into a single ast command that follows the same input pattern as exec/trace, so you can seamlessly switch between commands without changing arguments.

Notes

• Reuses run_common::load_source and run_common::resolve_lang instead of duplicating
• Removes --spans flag (only --raw kept for simplicity)