Complete rewrite of Hector based on Google's ADK-Go patterns and native A2A protocol integration.

Key changes:
- Native A2A protocol using github.com/a2aproject/a2a-go
- Interface-first architecture aligned with ADK-Go
- Iterator pattern (iter.Seq2) for event streaming
- Built-in Chain-of-Thought reasoning loop
- Workflow agents (Sequential, Parallel, Loop) for composability
- Three-layer memory system (Session, Working, Long-term)
- Config-first approach with YAML configuration
- Server-only CLI with validation foundation
- Comprehensive observability (OpenTelemetry + Prometheus)
- Rate limiting with multiple time windows
- TLS configuration for enterprise deployments
- Builder patterns for fluent programmatic API

Modules ported:
- Agent (llmAgent, remoteAgent, workflowAgent)
- Tools (file, command, search, web, MCP, agent-as-tool)
- Session (in-memory and SQL stores)
- Memory (working memory strategies, vector/keyword indexing)
- Config (YAML loader, providers, validation)
- Runtime (factory pattern, multi-pass agent building)
- Server (HTTP/gRPC, A2A protocol handlers, executor)
- Task (state machine, HITL, persistence)
- Observability (tracing, metrics)
- Auth (JWT validation, middleware, interceptors)
- HTTPClient (retry, backoff, TLS)
- RateLimit (token/request limits, SQL/memory stores)
- Logger (structured logging, filtering, colored output)
- Utils (token counting, filesystem helpers)

Modules excluded/replaced:
- A2A/Transport/Protocol: Offloaded to a2a-go
- AGUI: Excluded for leanness
- Plugins: Deferred until stable release
- Reasoning: Replaced by built-in CoT Flow pattern
- Component: Replaced by Runtime factory pattern

All modules reviewed and verified for correct porting and adoption.