Fix dumpconfig's non-safe YAML and add a --stdout option

[mkarlesky]

Summary

• Fix: clean, safe YAML from dumpconfig. FilenameExtension (lib/ceedling/filename_extension.rb) had no custom YAML serialization hook, so a default (--app enabled) dumpconfig run emitted !ruby/object:FilenameExtension tags into :extension config entries — unreadable by any safe YAML loader, including Ceedling's own (permitted_classes: [Symbol] only). FilenameExtension#encode_with now dumps it as a plain YAML sequence of its own extensions, matching its internal data exactly. It's the only custom object type Ceedling puts into the config hash, so this is an isolated fix, not a broader pattern.

• New: dumpconfig --stdout. Writes the resolved YAML directly to standard output instead of a file, for a consuming tool that wants Ceedling's configuration without a temp file. FILEPATH becomes optional; when --stdout is set, every positional argument is treated as a SECTIONS path element instead, and every console banner/notice dumpconfig would otherwise print is suppressed so the stream stays pure YAML.

Tests

• spec/units/filename_extension_spec.rb — new in-memory case proving YAML.dump produces no !ruby/object tag and round-trips cleanly through YAML.safe_load.
• New spec/system/dumpconfig_spec.rb — covers a real (--app default) file dump's clean :extension output, --stdout with and without a SECTIONS filter, and the FILEPATH-required error path when --stdout is omitted.

Docs

Updated docs/mkdocs/getting-started/command-line.md (flag table + FILEPATH/SECTIONS explanation). docs/mkdocs/reference/command-line.md's brief link-back summary for dumpconfig already covers this without changes.

Verification

Full unit + integration suites and the new/adjacent system specs (dumpconfig_spec.rb, mixin_ordering_spec.rb, ruby_replacement_spec.rb) green on host (macOS) and the madsciencelab-plugins Docker image (Linux).

🤖 Generated with Claude Code