Canonical agent enum shared by every subsystem that targets the
six AI dev agents (Claude Code, Cursor, Codex, Windsurf, Cline,
Continue). Review item 19 (maint P1) — previously three near-
identical enums (ai_hooks::AgentTarget, mcp_register::McpAgentTarget,
skills::SkillAgent) carried the same six variants and the same
slug mapping, with only per-subsystem method bolt-ons differing.
Shared HTTP / network primitives. Currently a single-module wrapper
around ureq::Agent with timeouts, but the right home for any future
cross-cutting network helpers (URL allowlist, redirect callback, etc.).