feat(docs): generate Section 5 from function comment headers

The man-page-style comment header above each function in functions/*.fish
becomes the SSOT for that function's documentation. Writing a new function
and documenting it are now the same act.

- manualtools.parse_functions() parses every header carrying a # CATEGORY;
  absence of one is the opt-in, keeping bundled-plugin and prompt internals
  out of the manual with no exclusion list to maintain.
- build-manual.py generates entries for both --concat and --site, with
  **Dependencies:** rendered as links and a **Used by:** reverse index
  computed in one pass. Cross-category links are the navigation win.
- docs/manual/05-functions/*.md reduced to frontmatter-only stubs. Every
  intro measured zero words, so the category files were pure entry
  containers; ordering, titles, and helpKeywords routing are untouched.
- _first_sentence() unwraps the leading hard-wrapped paragraph and skips
  the whole Synopsis block, not just its label line. Site cards no longer
  truncate mid-clause or show a synopsis as their description.

Verification, per the design spec:
- test_concat_roundtrips_original scoped to sections 0-4 and 6-11. It
  guarded a format migration; this is a content migration.
- replaced by structural checks: one entry per categorised function, the
  required sections present, every category resolving to a stub with no
  stub empty, and every declared dependency resolving to a real function
  or a type -q-guarded binary.
- public functions lacking # CATEGORY warn rather than fail, so a new
  user-facing function going undocumented stays visible in CI.

24/24 checks pass. 94 entries generated from 94 parsed headers.

Also drops a stale claim from open-url's NOTES: config-help --html calls
xdg-open directly and has never called open-url.

docs/manual/05-functions/11-pager-and-logging.md

@@ -7,41 +7,4 @@ helpKeywords:
 - logging
 ---
 
-## logs
 
-    Synopsis:  logs [-c <category>]
-    Interactively browses terminal log files sorted newest-first using fzf.
-
-      -c/--category  Filter to: scrollback, paru, or yay
-
-    Keybindings inside the fzf browser:
-      Enter    Open in $PAGER
-      Ctrl+E   Open in $EDITOR
-      Ctrl+D   Delete (with confirmation)
-      ?        Toggle keybind help overlay
-
-    Paru and yay logs open in ov with syntax highlighting and sticky section
-    headers. Scrollback logs open in ov with per-command sticky prompt headers
-    based on OSC 133 markers.
-
-    logs
-    logs -c paru
-    logs -c scrollback
-
-## smart_exit
-
-    Synopsis:  smart_exit [-n]
-    Closes the shell session. In Kitty, captures the terminal scrollback to
-    a timestamped log file in $SCROLLBACK_HISTORY_DIR before exiting.
-    Automatically prunes the oldest logs when the count exceeds
-    $SCROLLBACK_HISTORY_MAX_FILES.
-
-      -n/--no-log  Exit without saving a scrollback log
-
-    The exit builtin is wired to smart_exit for interactive sessions.
-    Typing exit or Ctrl+D behaves identically to smart_exit.
-
-    smart_exit
-    smart_exit --no-log
-
----