ci: add coverage buffer and contributor test-quality enforcement

[JSONbored]

Parent: #32

Goal

Stop contributor PRs from hovering just below the 95% branch coverage gate.

Requirements

• Keep the global 95% coverage gate.
• Add contributor guidance that every behavioral change must include unit/integration tests for new branches, fallback paths, and sanitizer boundaries.
• Add a small coverage buffer target in docs/PR template: contributors should aim for 96%+ branch coverage locally so CI variance does not fail near the threshold.
• Improve CI output or docs so contributors know which files need branch coverage.
• Do not lower the threshold unless a maintainer explicitly changes policy later.

Acceptance Criteria

• PR template mentions npm run test:ci, 95% global coverage, and invariant/regression coverage.
• Feature issue template asks for test scenarios and public/private safety boundaries.
• CI still enforces 95% for lines, statements, functions, and branches.
• No code behavior changes.

Testing Requirements

This issue must preserve the repo's CI gate:

• npm run test:ci must pass.
• Global coverage must remain at or above 95% for lines, statements, functions, and branches.
• Add or update tests for every new branch, fallback path, sanitizer rule, and regression.
• Add invariant/property-style tests when behavior depends on sorting, gating, public/private boundaries, scoring, queue pressure, or source-upload safety.
• Public GitHub output must be tested against forbidden language: wallet, hotkey, raw trust score, payout, reward estimate, farming, private reviewability, and public score estimate.

Required Test Scenarios

• Existing npm run test:ci.
• Template/docs check if existing docs validation covers stale testing language.