Phase 2 — Normalize file naming (#59)

* docs(naming): drop scrum4me- prefix from doc filenames

Rename 10 docs/scrum4me-*.md files to unprefixed kebab-case names.
Update every internal link in docs/, CLAUDE.md, AGENTS.md, README.md.

* docs(naming): lowercase API.md and MD3 filenames

Rename docs/API.md → docs/api.md and
docs/MD3_Color_Scheme_Documentation.md → docs/md3-color-scheme.md.
Update all internal links across 7 files.

* docs(naming): rename plan file to kebab-case ASCII

Rename "docs/plans/Tweede Claude Agent — Planning Agent.md"
→ docs/plans/tweede-claude-agent-planning.md. No external links needed updating.

* docs(naming): rename middleware.md to proxy.md (next 16)

docs/patterns/middleware.md → docs/patterns/proxy.md following
the Next.js 16 proxy.ts rename. Update link in CLAUDE.md.

* docs(naming): polish CLAUDE.md doc-index after renames

Fix doubled scrum4me-scrum4me-mcp repo references (cascade from
prior sed) in CLAUDE.md, docs/architecture.md, backlog.md,
agent-instruction-audit.md, and plans/ST-1109. Update
'Middleware' label to 'Proxy middleware' in patterns table.

README.md

@@ -155,7 +155,7 @@ Verwacht: alle 69 tests slagen, 0 failures.
 bash scripts/test-api.sh
 ```
 
-De curl-tests dekken alle 7 API-endpoints: auth (401), demo-blokkering (403), inputvalidatie (400) en happy paths. Zie `docs/scrum4me-test-plan.md` voor het volledige testplan.
+De curl-tests dekken alle 7 API-endpoints: auth (401), demo-blokkering (403), inputvalidatie (400) en happy paths. Zie `docs/test-plan.md` voor het volledige testplan.
 
 ## Database
 
@@ -279,7 +279,7 @@ De productieomgeving is gericht op Vercel + Neon.
 
 ### Documentatie
 
-- [Functionele specificatie](docs/scrum4me-functional-spec.md)
-- [Technische architectuur](docs/scrum4me-architecture.md)
-- [Backlog](docs/scrum4me-backlog.md)
+- [Functionele specificatie](docs/functional.md)
+- [Technische architectuur](docs/architecture.md)
+- [Backlog](docs/backlog.md)
 - [Agent-instructie audit](docs/agent-instruction-audit.md)