refactor: reduce complexity of main in src/main.rs (#382)

Extract the Execute command arm (~120 lines of nested logic) into
dedicated helper functions:

- run_compile: handles Compile arm flags/dispatch
- run_execute: orchestrates the Execute pipeline (parse → context → run → memory → summary)
- build_execution_context: reads front matter into ExecutionContext, including OTel stats
- process_cache_memory: checks cache-memory tool config and processes memory artifacts
- print_execution_summary: formats and prints the final results table

Also flatten the top-level if-let/match in main to let-else + match,
reducing nesting depth.

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Author: github-actions[bot]