feat(markdown): add CommentType.markdown backed by tree-sitter-markdown

[HartmannNico]

Closes #94

Summary

Adds comment_type: markdown backed by tree-sitter-markdown, enabling <!-- @needs … --> HTML-comment markers in Markdown files to be ingested as traceability nodes by sphinx-codelinks.

Primary use case: AI agent and skill definition files (.claude/agents/*.md, .github/prompts/*.md) that are the actual implementation artefacts of agentic systems and need to participate in a requirements → architecture → design → implementation traceability chain.

Changes (3 files, ~24 lines)

source_discover/config.py

• Added "markdown": ["md", "markdown"] to COMMENT_FILETYPE
• Added markdown = "markdown" to CommentType enum

analyse/utils.py

• Added MARKDOWN_QUERY = """(html_block) @comment""" — tree-sitter-markdown maps standalone <!-- … --> blocks to html_block nodes
• Added elif comment_type == CommentType.markdown: branch in init_tree_sitter() importing tree_sitter_markdown
• No SCOPE_NODE_TYPES entry: Markdown has no function/class scopes; oneline-only mode never invokes scope association

pyproject.toml

• Added tree-sitter-markdown>=0.5.1

Caller note

Because html_block node text includes the full <!-- … --> delimiters, callers must configure end_sequence: " -->" (not the default "\n") in their oneline_comment_style. This is the only behavioural difference from all other comment_type entries.

Example conf:

"my_agents": {
    "source_discover": {"src_dir": ".claude/agents", "comment_type": "markdown"},
    "analyse": {
        "get_oneline_needs": True,
        "get_need_id_refs": False,
        "oneline_comment_style": {
            "needs_fields": [...],
            "start_sequence": "<!-- @needs ",
            "end_sequence": " -->",
        },
    },
}

Relation to other PRs

Analogous to PR #92 (bash). PR #82 (TypeScript) was the structural template.
This PR is independent of PR #92 — it is based directly on main and does not require bash support to be merged first.