Code review actions: rename version file, drop Gemfile conditional, tighten gemspec, switch URLs to GitHub

Three tightly-coupled cleanups from the Tobi+Sandi review:

1. Rename lib/opencode/rails/version.rb -> lib/opencode/rails_version.rb
   (Sandi S1: file path was lying about its contents — the file defines
   Opencode::RAILS_VERSION, NOT Opencode::Rails::VERSION). Updated the
   require_relative in opencode-rails.gemspec and lib/opencode-rails.rb
   to match. Removed the now-empty lib/opencode/rails/ directory.

2. Drop the 'if File.exist?(...) gem opencode-ruby, path: ...' Gemfile
   conditional (Tobi T2 / Sandi S4: Bundler behavior must not depend on
   filesystem state). The dev-time sibling-repo override is now
   documented in CONTRIBUTING.md as the standard 'bundle config
   local.opencode-ruby <path>' pattern, which is what Bundler ships
   for this use case.

3. Tighten opencode-ruby runtime dep from '~> 0.0.1.alpha1' to
   '= 0.0.1.alpha1' (Tobi T1: ~> during alpha is aspirational; pin
   exactly until the public API stabilizes).

Same commit also switches all forward-looking URLs (gemspec homepage,
metadata, README link to opencode-ruby) from Gitea to GitHub since the
gems will eventually publish there. Functional 'git:' URL in
ajent-rails' Gemfile stays on Gitea — that's where the gems actually
are right now; ajent-rails Gemfile flips to GitHub when the user does
the actual remote setup.

Test assertion in loading_test was tightened to match either an
installed-gem path ('gems/opencode-X-VERSION/') or a sibling-repo
checkout ('/opencode-X/'), via a small GEM_PATH_PATTERN helper.

15 tests, 50 assertions, 0 failures.

CONTRIBUTING.md

@@ -0,0 +1,71 @@
+# Contributing to opencode-rails
+
+## Running the test suite
+
+```bash
+bundle install
+bundle exec rake test
+```
+
+The smoke tests live in `test/opencode/`. They prove that:
+
+- Every gem-provided constant resolves
+- The opencode-ruby umbrella loads transitively
+- Source locations point at the right gem
+- The version constant is not under an `Opencode::Rails` module
+  (that would shadow `::Rails` in host apps; see comment in
+  `lib/opencode/rails_version.rb`)
+- Public API contracts on the AR-coupled classes hold (Session, Turn,
+  MessageArtifacts) — verified via `Method#parameters`, not behavior
+- Value objects (Artifact, SandboxFile, Transform, Impostor) round-trip
+  through their public interfaces
+
+Behavioral tests for AR + ActiveStorage paths live in the host app
+that originally produced this code (`ajaynomics/ajent-rails`'s
+`test/lib/opencode/rails/`). Same pattern as opencode-ruby.
+
+## Working on opencode-rails together with opencode-ruby
+
+opencode-rails depends on opencode-ruby. During development of either
+gem you frequently need changes in opencode-ruby to be picked up by
+opencode-rails without going through a release cycle.
+
+**Use Bundler's `local` config — not Gemfile conditionals.** Bundler
+behavior must never depend on filesystem state inside the Gemfile.
+
+```bash
+# Once per dev machine. Replace the path with wherever you have
+# opencode-ruby checked out.
+bundle config local.opencode-ruby /path/to/opencode-ruby
+
+# Then bundle install/update against the local copy:
+bundle install
+```
+
+To switch back to the released version:
+
+```bash
+bundle config --delete local.opencode-ruby
+bundle install
+```
+
+See [Bundler's documentation on local git overrides](https://bundler.io/v2.5/git.html#local).
+
+## Releasing
+
+This gem is in alpha. Versions ship as `0.0.x.alphaN` until the public
+API stabilizes.
+
+Coordinated releases with opencode-ruby:
+
+1. In opencode-ruby: bump `Opencode::VERSION`, tag, push.
+2. In opencode-rails: bump `Opencode::RAILS_VERSION`, update the
+   `add_runtime_dependency "opencode-ruby", "= X.Y.Z"` line in the
+   gemspec to match the new opencode-ruby version (alpha discipline:
+   pin exactly, not pessimistically). Tag, push.
+3. In any consumer (e.g., ajent-rails): bump both `tag:` lines in the
+   Gemfile to the new versions; `bundle update opencode-ruby opencode-rails`.
+
+## Reporting issues
+
+File at <https://github.com/ajaynomics/opencode-rails/issues>.