Allow marcel 2 alongside marcel 1

Companion to the same change in opencode-ruby. This gem depends on
activestorage directly, and Rails edge moved activestorage to marcel ~> 2.0,
so pinning marcel ~> 1.0 makes this gem unresolvable alongside edge:

  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.
  So, because every version of opencode-rails depends on marcel ~> 1.0,
    version solving has failed.

Found by bumping ajent-rails to the new opencode-ruby ref and re-resolving:
fixing opencode-ruby alone moved the conflict here rather than clearing it,
because both gems carried the same pin.

A permissive range rather than a bump, for the same reason as the sibling gem:
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 range.

One call site, lib/opencode/sandbox_file.rb:51:

  Marcel::MimeType.for(name: basename)

Verified identical under marcel 2.1.0 rather than assumed, including the
no-extension fallback this code relies on:

  for(name: "a.png") -> "image/png"
  for(name: "x.pdf") -> "application/pdf"
  for(name: "n.txt") -> "text/plain"
  for(name: "noext") -> "application/octet-stream"

94 runs, 382 assertions, 0 failures on Ruby 4.0.6. CI covers 3.2 through 4.0,
which exercises both ends of the range.

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

opencode-rails.gemspec

@@ -38,7 +38,13 @@ Gem::Specification.new do |spec|
   # exactly (= not ~>) so that consumers always pick the version this gem
   # was tested against.
   spec.add_runtime_dependency "opencode-ruby", "= 0.0.1.alpha9"
-  spec.add_runtime_dependency "marcel", "~> 1.0"
+  # Rails edge moved activestorage to marcel ~> 2.0, and this gem depends on
+  # activestorage directly, so pinning ~> 1.0 makes it unresolvable alongside
+  # 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. The single call site,
+  # Marcel::MimeType.for(name:), behaves identically in both majors.
+  spec.add_runtime_dependency "marcel", ">= 1.0", "< 3"
 
   # Rails sub-libraries used at runtime. Depending on these individually
   # (instead of the `rails` umbrella) avoids forcing host apps to load