docs: Add architecture decision records

[E-Conners-Lab]

Adds three ADRs under docs/adr/ capturing real design choices already in the code:

• ADR-0001 Modular tool registry: per-module TOOLS lists aggregated by _build_registry in mcp_tools/__init__.py, with duplicate-name detection raising at import time.
• ADR-0002 Auth and metrics wrapping at registration: the single registration loop in network_mcp_async.py wrapping every tool with track_tool_call then auth_enforced, resolved once at registration time.
• ADR-0003 Fail-loud error handling: the APIError (4xx, safe) vs InternalError (5xx, never exposed) split and safe_error_response with correlation IDs in core/errors.py.

Documentation only, no code changes.