ADR docs: create docs/adr/ directory with template and index

[mwaldheim]

Architecture Decision Records document significant decisions, context and consequences.

Tasks:

• docs/adr/README.md - what ADRs are, how to create one
• docs/adr/template.md - standard ADR template
• docs/adr/ADR-001-plugin-transport-grpc.md (see ADR-001: Out-of-process plugin transport via hashicorp/go-plugin + gRPC #88)
• docs/adr/ADR-002-plugin-discovery-path.md
• docs/adr/ADR-003-apache2-license.md
• Link ADR index from docs/architecture.md and CONTRIBUTING.md

[mwaldheim]

Progress update (2026-05-23):

• docs/adr/README.md, template, and ADR index updates are in PR docs(adr): ADR-002 - Repository Separation Plan #104.
• ADR-002 and ADR-003 are included in PR docs(adr): ADR-002 - Repository Separation Plan #104.

Status: keep open until PR #104 merges into main, then this can likely be closed.