docs(adr): add ADR 0001 for SDK API design principles (#144)

[mubking]

Add ADR for SDK API design principles

Closes #144

Summary

Adds an Architecture Decision Record documenting the SDK's API design principles, so future contributions and reviews have a shared reference instead of re-deciding these questions case by case. This is a documentation-only change: it records current practice and introduces no code changes.

What it adds

• docs/adr/0001-api-design-principles.md — the ADR itself, covering the five areas the issue calls for:
  • Naming — camelCase action verbs grouped by domain module, a single package entry point, and the doThing / safeDoThing / enhancedDoThing / safeEnhancedDoThing prefix convention that communicates a function's error behavior from its name.
  • Error handling — the single PocketPayError(message, code, details?, cause?) class with stable machine-readable codes, and the two consumption styles (throwing base functions vs. safe* functions returning a { ok, value } / { ok, error } discriminated union).
  • Configuration — centralized SDKConfig, resolved through resolveConfig with a clear override → environment → default precedence, and fail-fast validation.
  • Async operations — network operations return promises; pure helpers stay synchronous; enriched variants carry non-fatal ResultWarnings and actionable RecoveryHints.
  • Backwards compatibility — SemVer, changelog discipline, additive change preference, and the supported Node/TypeScript/Stellar SDK baselines.
• docs/adr/README.md — an ADR index describing what ADRs are and listing the records.
• README link — the Documentation section now links the ADR directory.

Acceptance criteria

• An ADR is added for SDK API design principles
• The ADR covers naming, errors, config, async behaviour, and compatibility
• The README links to the ADR
• The ADR is written clearly for contributors

Notes

The ADR documents the API patterns as they exist in the current source (the result union, PocketPayError shape, the safe*/enhanced* naming family, Partial<SDKConfig> overrides, and the ResultWarning/RecoveryHint enrichment). No source files were modified.

Separately, while preparing this ADR I noticed main currently does not type-check: src/types/index.ts is missing exports that many modules import from ../types (PocketPayError, SDKConfig, StellarNetwork, SuccessResult, PocketPayResult, and others), so tsc --noEmit fails project-wide. That is a pre-existing issue unrelated to this documentation change and is not addressed here; I'd suggest tracking it as its own bug.