feat(cli): add per-session internal logging

Summary

• Adds a per-invocation internal tracing session with a UUIDv7 log file, using non-blocking JSONL writes so diagnostic logging stays off stdout and stderr.
• Stores session logs in platform log locations: XDG state directories on Linux, ~/Library/Logs/Once on macOS for Console.app visibility, and LOCALAPPDATA on Windows.
• Preserves existing stderr verbosity behavior while adding internal startup, completion, failure, and argument-parse early-exit events.
• Documents what contributors and agents should log, including cache decisions, action digests, provider choices, retries, durations, and failure causes without secrets.

Testing

• mise exec -- cargo fmt --all -- --check
• mise exec -- cargo check -p once-cli
• mise exec -- cargo clippy -p once-cli --all-targets -- -D warnings
• mise exec -- env HOME=<temporary directory> cargo run -q -p once-cli -- --list
• mise exec -- env HOME=<temporary directory> cargo run -q -p once-cli -- --help

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