docs: add delivery planning and execution system

[MartinKalema]

What changed

• Added a delivery-assurance guide covering the work needed beyond sound architecture.
• Added an engineering planning and estimation guide that starts from the complete outcome instead of a coding task list.
• Added an execution-management guide for deadlines, backward planning, dependencies, capacity, transitions, variance, and client communication.
• Added a human-centered systems guide that prevents planning and measurement from treating people as interchangeable resources.
• Updated the Engineering Design System to connect design, planning, execution, release, operation, and learning.
• Linked the new guides from the README.

Why

The existing system helped engineers identify the correct outcome, requirements, invariants, authority, and minimum architecture. It did not fully explain how to manage that work through time until a high-quality client outcome is delivered.

These guides add that missing execution layer. They also define an important human boundary: deadlines and planning failures must not be preserved by silently consuming engineers' health, dignity, agency, or personal time.

Impact

This is a documentation-only change. It does not change application behavior, dependencies, database schemas, CI configuration, or deployment infrastructure.

Validation

• Verified Markdown formatting with git diff --check.
• Verified that the local Markdown links in all changed documents resolve.
• Built the PR from a clean branch based on the current main branch.
• Confirmed that only the six intended documentation files are included.