Add a CliTests case for `okf graph` plain-text mode (no `--dot`)

[jchable]

Difficulty / est. effort: ~30min, small

Context: tests/OKF4net.Tests/CliTests.cs covers okf graph --dot extensively (Graph_dot_prints_digraph, Graph_dot_styles_broken_links_dashed_and_red, Graph_dot_does_not_style_resolvable_links) but has no test for the default plain-text mode. CmdGraph's else branch in src/OKF4net.Cli/OkfCli.cs (around lines 358-376) prints each concept id followed by its outgoing links, marked -> for a resolvable link and -x for a broken one — this behavior is currently unverified by any test.
Files to touch: tests/OKF4net.Tests/CliTests.cs, src/OKF4net.Cli/OkfCli.cs (read-only, for reference)
What to do:

1. Add a new [Fact] in tests/OKF4net.Tests/CliTests.cs near the existing Graph_dot_* tests (around line 119), e.g. Graph_plain_text_prints_arrows_and_broken_link_markers.
2. Using TempDir, build a small bundle with one concept linking to an existing concept and one linking to a non-existent target, similar to the setup in Graph_dot_styles_broken_links_dashed_and_red but without --dot.
3. Run okf graph <bundle> (no --dot flag).
4. Assert exit code 0, assert the output contains the source concept id on its own line, a line with the -> marker for the resolvable link, and a line with the -x marker for the broken link (matching the format built in CmdGraph: " {mark} {link.Target}\n").
   How to verify: dotnet test OKF4net.sln --filter "FullyQualifiedName~CliTests" — expect the new test and the full CliTests class to pass.
   Good to know: See CONTRIBUTING.md for build/test basics. The README's ### As a CLI section documents graph as printing "the cross-link graph (--dot for Graphviz DOT)" — this issue covers the non---dot default.

[ahmdkaml]

I want to work on this issue. Can you assign it to me?