📝 docs: add section deletion cookbook and cross-link section ops

Documents the section::filter_sections() + section::collect() pattern
for deleting a Markdown section by heading, which had no example in
docs/ or the processing-markdown skill despite being achievable with
existing built-ins.

Author: harehare

docs/books/src/start/example.md

@@ -391,6 +391,51 @@ Use the tool like this.
 
 **Output**: `["Introduction", "Usage"]`
 
+### Delete a Section by Heading
+
+Use `filter_sections()` with a negated predicate to drop sections whose title matches, then flatten back to Markdown with `collect()`:
+
+```bash
+$ mq -A 'section::filter_sections(fn(s): section::title(s) != "Deprecated";) | section::collect()' README.md
+```
+
+Or with `nodes`:
+
+```mq
+import "section"
+| nodes
+| section::filter_sections(fn(s): section::title(s) != "Deprecated";)
+| section::collect()
+```
+
+**Input example**:
+
+```markdown
+# Introduction
+
+Welcome to the project.
+
+## Installation
+
+Run the following command.
+
+## Deprecated
+
+Do not use this anymore.
+```
+
+**Output**:
+
+```markdown
+# Introduction
+
+Welcome to the project.
+
+## Installation
+
+Run the following command.
+```
+
 ## Table Operations
 
 The table module provides functions for extracting and transforming Markdown tables.

docs/llms-full.txt

@@ -707,6 +707,9 @@ mq -A 'section::sections() | section::by_level(2)' README.md
 
 # Generate table of contents
 mq -A 'section::sections() | section::toc()' README.md
+
+# Delete a section by heading
+mq -A 'section::filter_sections(fn(s): section::title(s) != "Deprecated";) | section::collect()' README.md
 ```
 
 ## Table Operations

skills/processing-markdown/EXAMPLES.md

@@ -43,6 +43,20 @@ mq --csv 'include "csv" | csv_parse(true) | csv_to_markdown_table()' data.csv
 mq -I raw 'identity()' data.json                            # Disable auto-parse
 ```
 
+## Section Operations
+
+Split/filter a document into sections by heading, then flatten back to Markdown with `section::collect()`. Needs all document nodes: use `-A` or pipe through `nodes`.
+
+```bash
+mq -A 'section::section("Installation")' README.md                                  # Extract section by title
+mq -A 'section::section("Installation") | section::bodies() | first()' README.md    # Section body only (no header)
+mq -A 'section::sections() | section::by_level(2)' README.md                        # h2 sections only
+mq -A 'section::sections() | section::toc()' README.md                              # Table of contents
+mq -A 'section::filter_sections(fn(s): section::title(s) != "Deprecated";) | section::collect()' README.md  # Delete a section by heading
+```
+
+See [example.md](../../docs/books/src/start/example.md) for the full section reference (import/include forms, `-A` vs `nodes`, more filters).
+
 ## Multi-File & Aggregation
 
 ```bash

skills/processing-markdown/SKILL.md

@@ -102,6 +102,10 @@ mq -I html 'identity' page.html           # HTML → Markdown
 
 # Streaming large files
 mq --stream 'select(contains("ERROR"))' large.md
+
+# Section operations (needs -A or `nodes` — see EXAMPLES.md)
+mq -A 'section::section("Installation")' file.md                                                             # Extract section by heading
+mq -A 'section::filter_sections(fn(s): section::title(s) != "Deprecated";) | section::collect()' file.md    # Delete section by heading
 ```
 
 ## HTML Input: Always Use Markdown Selectors