docs: initialize architecture decision records

[supermomonga]

Summary

Migrate architectural decisions from informal design documents into a structured Architecture Decision Record (ADR) system using MADR format.

• Add adrs.toml configuration for ADR management
• Create 8 ADRs documenting key design decisions (0001-0008)
• Consolidate scattered rationale from deleted documents (00-solution-choice.md, 09-implementation-plan.md) into decision records
• Update existing specification documents with references to relevant ADRs
• Reorganize docs/README.md to highlight ADR index and current architecture

Key ADRs

1. ADR 0002: ACP adapter client-independent, UI delegated to external clients
2. ADR 0003: ZCode official host service launched via bundled Electron runtime
3. ADR 0004: Stateful protocol conversion between ACP and ZCode
4. ADR 0005: Support only latest verified ZCode 1 version with strict compatibility
5. ADR 0006: Lock ACP v1 contract, separate future protocol versions
6. ADR 0007: Maintain native permission, input, and credential boundaries
7. ADR 0008: Build all platform adapters on Linux runner only

Testing

• Verify all ADR documents render correctly as Markdown
• Confirm cross-references between ADRs and specification documents resolve correctly
• Check that adrs.toml configuration is valid and compatible with adrs CLI
• Validate that deleted documents' rationale is captured in corresponding ADRs
• Test that docs/adr/README.md index links to all ADR files