Support sub-attribute selection for structured attributes in publish configuration

[opt12]

Closes: #829

---

Summary

The publish list in .doorstop.yml previously only supported simple string attribute names. Publishing structured attributes (list of dicts) resulted in a raw Python object dump in the output table. This MR extends the publish configuration to allow selecting specific sub-attributes for rendering, including hyperlink support with combined labels.

---

Changes

doorstop/core/publishers/markdown.py

• Added _parse_publish_entry() – parses publish list entries; backward compatible: string entries behave as before, dict entries use the attribute name directly as key with a fields configuration
• Added _render_fields() – renders selected sub-attributes of structured attributes as Markdown links or plain text; supports single-field labels (url: section), combined multi-field labels (url: {label: [file, section], separator: ": "}), and plain text fields; multiple entries joined with <br>
• Modified _lines_markdown():
  • publish list now supports both string and dict entries
  • plain list attributes joined with <br> instead of raw Python list dump
  • trailing yield "" only emitted when table was actually printed

docs/reference/item.md

• Minor wording fix: removed redundant "key and value" in existing sentence
• Added documentation for list attribute rendering (<br> separator)
• Added new section "Publishing sub-attributes of structured attributes" with syntax reference, fields format table, single-field and combined-label examples

doorstop/core/publishers/tests/helpers.py

• Added YAML_STRUCTURED_ATTRIBUTES – document config with structured spec-refs-from publish entry
• Added YAML_COMBINED_LABEL_ATTRIBUTES – document config with combined label publish entry
• Added YAML_LIST_ATTRIBUTE – document config with plain list attribute
• Added YAML_INVALID_PUBLISH_ENTRY – document config with attr: ~ for edge case testing

doorstop/core/publishers/tests/test_publisher_markdown.py

• Added import of MarkdownPublisher
• Added TestParsePublishEntry – 5 unit tests for _parse_publish_entry()
• Added TestRenderFields – 14 unit tests for _render_fields(), including combined label and separator variants
• Added TestPublishLinesCustomAttributesExtended – 8 integration tests covering all new code paths in _lines_markdown()

---

New .doorstop.yml syntax

attributes:
  publish:
    - simple-attribute          # existing behavior – unchanged
    - spec-refs-from:           # structured attribute
        fields:
          - url: section        # single field as link text
    - spec-refs-to:
        fields:
          - url:
              label: [file, section]   # combined label
              separator: ": "          # optional, default: ": "

---

Backward Compatibility

Fully backward compatible. All existing string entries in publish behave exactly as before.

---

Testing

867 existing tests pass. 25 new tests added.