Fix module path so v2 releases are installable

[polymorcodeus]

Summary

Fixes the Go module path so tagged v2 releases can actually be installed. Every v2 tag from v2.0.0 through v2.4.0 shipped with module github.com/polymorcodeus/lnk, which is missing the /v2 suffix Go requires at major version 2 and up. That oversight made every v2 tag uninstallable (invalid version: module contains a go.mod file, so module path must match major version) and left go install ...@latest silently resolving to v1.5.0. This PR corrects it by moving the module to github.com/polymorcodeus/lnk/v2.

What's new

• Module path - github.com/polymorcodeus/lnk becomes github.com/polymorcodeus/lnk/v2, matching the major-version suffix requirement.
• Import paths - every internal import across cmd, service, internal, and tools (133 references across 66 files) is updated to the /v2 prefix.
• Install docs - the README install command is updated to go install github.com/polymorcodeus/lnk/v2@latest.

Behavior changes / guardrails

• The bare github.com/polymorcodeus/lnk@latest path keeps resolving to v1.5.0; v2 installs now use github.com/polymorcodeus/lnk/v2@latest.
• Existing tags v2.0.0 through v2.4.0 remain uninstallable because they point at commits with the old path; the first installable v2 release is v2.4.1.

Testing

• make check runs fmt, vet, lint, unit, and integration suites; all green.