fix(config_help): reliable section lookup and jump via index + tail-slice

- Add docs/fish-config.index mapping 100+ keyword aliases to exact heading
  text; config_help looks up here first, falls back to normalized heading
  scan (strips non-alphanumeric, lowercases both sides) for unknown terms.
- Resolve line numbers at runtime with grep -F so the index never goes
  stale from doc edits — only heading renames require an update.
- Fix section jump: --section-header pins delimiter lines as sticky headers
  removing them from ov pattern-search scope. Replace --pattern with
  tail -n +$start_line piped before ov; the target section is the first
  line so no search is needed and forward section nav still works.
- Fix empty sections sidebar: ov matches --section-delimiter against
  logical (ANSI-stripped) text, so "^#" is correct for bat-colored output;
  the previous ANSI escape-sequence regex never matched anything.

docs/fish-config.md

@@ -1216,7 +1216,16 @@ calls fisher update as its first step.
     config_help
 
 ov renders the Markdown with syntax highlighting and section-based
-navigation. Use , and . to jump between sections. Press / to search.
+navigation.
+
+    Space       next section
+    ^           previous section
+    Alt+u       toggle section list sidebar
+    /           search forward
+    n / N       next / previous search match
+    g           go to line number
+    j           interactive jump target (line, %, or 'section')
+    q           quit
 
 ## With bat