docs: Enhance document routing and create tech debt registry (PR 3a)

Strengthen documentation coherence and navigation:

README.md:
- Add 'Developer's Document Guide' section at top
- Link to CLAUDE.md, AGENTS.md, architecture, guardrails, validation gates
- Reference docs/INDEX.md and TECH_DEBT_REGISTER.md

docs/INDEX.md (new):
- Central documentation index with purpose and audience
- Core guides, executive/planning, tech debt, reference docs
- Quick links for first change, architectural decisions, validation

TECH_DEBT_REGISTER.md (new):
- Tech debt tracking per AGENTS.md v16.0
- 8 registered debts (code analysis suppressions, deferred refactoring)
- Impact/Effort matrix with paydown strategy (quarterly 20% target)
- Status tracking with completion history
- How-to guide for resolving debt

CLAUDE.md:
- Update TECH_DEBT_REGISTER reference from 'to be created' to actual link

Result:
- Developers land on README → directed to CLAUDE.md or AGENTS.md
- All docs cross-reference each other for circular navigation
- Tech debt fully visible and tracked with clear paydown roadmap
- docs/INDEX.md provides comprehensive reference point

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -558,7 +558,7 @@ Every suppressed rule, deferred refactor, and architectural compromise is debt.
 
 ### Tech Debt Registry
 
-Location: `TECH_DEBT_REGISTER.md` (to be created in PR 4)
+Location: [`TECH_DEBT_REGISTER.md`](TECH_DEBT_REGISTER.md)
 
 Format:
 ```