SI-28: Extract torrust-server-lib to standalone repository (EPIC #1669)

[josecelano]

Subissue of EPIC #1669 — Overhaul: Packages

Goal

Extract the torrust-server-lib crate from the tracker workspace into its own standalone repository.

Background

torrust-server-lib (folder packages/server-lib) is a shared utility crate for all Torrust HTTP servers. Key facts:

• Independent dependency tree: its only Torrust dependency is torrust-net-primitives (published v0.1.0).
• Five workspace consumers: axum-health-check-api-server, axum-http-server, axum-rest-api-server, axum-server, udp-server.
• Already published on crates.io as torrust-server-lib.

Scope

• Create standalone repository torrust/torrust-server-lib.
• Move source with history preservation.
• Make Cargo.toml self-contained.
• Set up CI.
• Update all 5 consumers + root Cargo.toml to use crates.io version dep.
• Remove from workspace members.
• Update AGENTS.md, packages/AGENTS.md, docs/packages.md.

Acceptance Criteria

• Standalone repo exists on GitHub with CI.
• No path dep on torrust-server-lib remains in workspace.
• cargo build --workspace, cargo test --workspace, linter all pass.