Add requirement granularity guidance to spec template, AGENTS.md section, and README

[Copilot]

Agents drafting first-pass feature specs tend to over-decompose into 20+ MUST requirements, making the coverage/review cycle disproportionately heavy before a team has decided which behaviors are core product promises. This adds guidance nudging toward 3–8 workflow-level requirements per feature.

Changes

• specs/REQ-010-granularity-guidance.md — new spec formalizing the granularity guidance requirements (REQ-010.1–3)

• src/init.ts — AGENTS_MD_SECTION — new "Requirement granularity" paragraph with:

  • 3–8 MUST recommendation with workflow-level vs. implementation-step contrast
  • Spec sizing smells: >10 MUSTs before tests, requirements restating internal steps, "MUST cover"/"MUST test" anti-patterns

• src/init.ts — SPEC_TEMPLATE — restructured from a generic single section to four explicit sections:

  ### REQ-001.1: Core workflows          ← MUST requirements for user workflows
  ### REQ-001.2: Safety and compatibility invariants
  ### REQ-001.3: Manual acceptance criteria   ← [manual]-tagged
  ## Notes and non-goals                ← prose, outside Requirements block

• README.md — new ## Requirement granularity section with guidance, good/bad examples, sizing smells checklist, and starter spec structure

• tests/granularity.test.ts — four tests covering REQ-010; each assertion verified by fresh-context reviewers to be genuinely falsifiable (e.g., REQ-010.2.1 extracts and checks that the Core workflows section contains a MUST, not just that the heading exists)

• Fixes Add guidance on manageable requirement granularity #4