Parent: #6853
Related: #6856

Objective

Implement the P3 runtime utility layer for extension operational ergonomics.

Proposal IDs covered

• P3-1 Shell detection and execution
• P3-2 Atomic file operations
• P3-3 Tool discovery and PATH management
• P3-4 Interactive TUI support
• P3-5 Cross-platform process detection

In scope

• Cross-platform utility APIs with explicit platform behavior documentation.
• Edge-case handling for TTY/process/path/file interactions.
• Usage examples for extension maintainers.

Out of scope

• P1/P2 helper families and docs-only lane.

Deliverables

• API boundary + platform support matrix.
• Implementations for all P3 helper areas.
• Tests for failure/edge behavior on supported platforms.
• Extension-facing examples.

Verification checklist

• Platform differences documented (Windows/macOS/Linux behavior).
• No silent failures in process/TTY/path handling.
• mage preflight passes.

Definition of done

• P3-1..P3-5 are complete, tested, and documented.