Allow marcel 2 alongside marcel 1

Rails edge moved activestorage to marcel ~> 2.0. This gem declared
marcel ~> 1.0, so an application that tracks Rails edge and loads both this
gem and opencode-rails can no longer resolve at all:

  Because every version of activestorage depends on marcel ~> 2.0
    and every version of opencode-rails depends on activestorage >= 7.1, < 9.0,
    every version of opencode-rails requires marcel ~> 2.0.
  And because every version of opencode-ruby depends on marcel ~> 1.0,
    every version of opencode-ruby is incompatible with opencode-rails >= 0.

That is ajent-rails today: it pins both gems by SHA and tracks
rails/rails main, and version solving fails outright.

This is a permissive range rather than a bump, deliberately. marcel 2 requires
Ruby >= 3.3 while this gem supports >= 3.2 and tests 3.2 in CI, so pinning to
2.x would silently narrow the supported Ruby range. With ">= 1.0", "< 3",
consumers on 3.3+ resolve marcel 2 and consumers on 3.2 resolve marcel 1.

There is one call site, lib/opencode/response_parser.rb:135:

  Marcel::MimeType.for(extension: File.extname(filename))

Verified identical under marcel 2.1.0 rather than assumed:

  for(extension: ".png") -> "image/png"
  for(extension: ".pdf") -> "application/pdf"

55 runs, 178 assertions, 0 failures on Ruby 4.0.6. CI covers 3.2 through 4.0,
which is what actually exercises both sides of the range.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rch35u85D3buqx7wEsCVki

opencode-ruby.gemspec

@@ -33,7 +33,13 @@ Gem::Specification.new do |spec|
   # ActiveSupport supplies the small set of core extensions used by the
   # client without pulling in Rails. Marcel identifies artifact content types.
   spec.add_runtime_dependency "activesupport", ">= 6.1", "< 9.0"
-  spec.add_runtime_dependency "marcel", "~> 1.0"
+  # Rails edge moved activestorage to marcel ~> 2.0, and this gem is loaded
+  # alongside it in applications that track edge. marcel 2 requires Ruby >= 3.3
+  # while this gem supports >= 3.2, so the bound is a permissive range rather
+  # than a bump: consumers on 3.3+ resolve marcel 2, consumers on 3.2 resolve
+  # marcel 1, and neither is forced. The single call site,
+  # Marcel::MimeType.for(extension:), is identical in both majors.
+  spec.add_runtime_dependency "marcel", ">= 1.0", "< 3"
 
   # minitest 6 removed minitest/mock; Object#stub now ships in the extracted
   # minitest-mock gem, which the suite uses to stub Net::HTTP, File and Tempfile.