docs: add MECE classification principle

[dapi]

What changed

• Added MECE as a governance principle for closed classifications.
• Defined boundaries for cross-references, orthogonal axes, open lists, and multi-upstream dependency graphs.
• Listed the Wikipedia MECE article and the official MADR site under the README methodology sources.

Why

MECE complements SSoT by requiring classifications to be both non-overlapping and complete within an explicitly declared scope. The limitations prevent the rule from being applied where graph relationships or multiple independent dimensions are intentional. The MADR link makes the methodology source behind the local ADR template discoverable from the repository overview.

Impact

Downstream projects receive the new principle through the canonical template during their normal Memory Bank update flow. The protected project-local managed copy remains unchanged until a lock-aware update. Repository readers can now reach both the MECE and MADR methodology sources from the root README.

Validation

• memory-bank-cli lint --scope-root template/memory-bank --entrypoint template/memory-bank/README.md
• memory-bank-cli doctor --profile template
• ruby tools/validate-priming-manifests.rb template/memory-bank
• git diff --check