v0.1.14 — DIFFERENTIATORS section + PURPOSE requires business goals

Two changes driven by real-use feedback:

1. PURPOSE now requires business goals.
A gh-style marketing tagline (single sentence describing what the project does) was passing the old 'substantive' check and being preserved by revive suggest. But it doesn't tell an agent WHY the project exists or what success looks like. Updated spec requires all three: what + why (business goal / success criterion) + essential philosophy or hard constraint. Marketing taglines now classify as placeholder and get regenerated.

2. New STATIC section: DIFFERENTIATORS.
Captures competitive positioning and deliberate design choices that contradict obvious alternatives. Prevents agents from suggesting 'why not use X?' for an X you already evaluated and rejected.

Format: <alternative> → <our choice/rationale> or unlike X, we Y because Z.

Brief layout: PURPOSE → DIFFERENTIATORS → INVARIANTS → GOTCHAS.

Full integration: init scaffolds placeholder, init --force preserves user edits, show suppresses placeholder bullets, suggest generates as Deliverable 2.

100/100 bats tests pass (+7 new cases).