Move strictmiddleware/echo/v5/ to strictmiddleware/echo-v5/ with its
own go.mod requiring Go 1.25, so the root module stays at Go 1.20.

This avoids forcing all runtime users to upgrade to Go 1.25 just
because echo v5 support was added. The echo-v5 package is a separate
Go module with only the echo v5 dependency.

Also changes package name from `v5` to `echo` to follow Go convention
(matching how github.com/labstack/echo/v5 uses `package echo`).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

The echo-v5 strict middleware requires Go 1.25+, which is ahead of the
rest of the runtime module (Go 1.20). To support this as a separate
child module, the CI and build tooling needed to become multi-module
aware.

- Replace four separate workflow files (ci.yml, lint.yml, tidy.yml,
  generate.yml) with a single ci.yml that calls the shared reusable
  workflow from oapi-codegen/actions v0.5.0
- Update the root Makefile to iterate child modules via
  git ls-files '**/*go.mod', matching the pattern used by oapi-codegen
- Bump golangci-lint from v1.55.2 to v2.10.1 and update flags for v2
- Add tidy-ci target for the reusable workflow
- Add strictmiddleware/echo-v5/Makefile with a Go version guard that
  skips gracefully when running on Go < 1.25

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>