feat: add shared backend-typed commands

[passcod]

🤖 Starts the process-wrap 11 command redesign with one generic command implementation selected by the blocking or Tokio backend marker.

• re-exports ergonomic std::Command and tokio::Command aliases while retaining CommandWrap compatibility
• keeps inferred with_new closure bodies on a process-wrap-owned command facade
• tracks portable arguments, environment, and cwd so each spawn gets a fresh native attempt
• preserves exact native commands behind explicit native-only conversion and escape paths
• preserves ordered regular/raw Windows arguments and case-insensitive Windows environment semantics
• marks the crate as 11.0.0 so the intentional public type transition is checked as a major change
• records the remaining provider and PTY integration design in the repository plan

Alternate spawn providers and PTY transport follow in stacked changes; this pull request establishes the shared command state and frontend boundary they consume.

🤖 Generated with Claude Code