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/08-terminal-management.md

@@ -7,54 +7,4 @@ helpKeywords:
 - terminal-mgmt
 ---
 
-## tab
 
-    Synopsis:  tab [args...]
-    Opens a new tab in Kitty (kitty @ launch --type=tab), WezTerm
-    (wezterm cli spawn), or Konsole. Uses current working directory,
-    or $cdto if set.
-
-    tab
-
-## split
-
-    Synopsis:  split [-h|-v] [command...]
-    Opens a new pane in Kitty or WezTerm, optionally running a command.
-
-      -h/--horizontal  (default) Split below
-      -v/--vertical    Split to the right
-
-    split
-    split -v nvim README.md
-
-## spwin
-
-    Synopsis:  spwin [args...]
-    Spawns a new terminal OS window in Kitty (via spawn-window.sh or
-    kitty @ launch --type=os-window) or WezTerm (wezterm cli spawn --new-window).
-
-## detach
-
-    Synopsis:  detach [-h] [--version] <command> [args...]
-    Runs a command fully detached via nohup with stdout/stderr discarded.
-    The command survives the current session.
-
-    detach rsync -a ./data remote:/backup/
-
-## bkg
-
-    Synopsis:  bkg <command> [args...]
-    Launches a command in the background via nohup with output discarded.
-    Simpler than detach; no version flag.
-
-    bkg firefox
-
-## ssh
-
-    Synopsis:  ssh [args...]
-    In Kitty, wraps ssh with kitten ssh for better terminal integration
-    (multiplexing, copy/paste support). Falls back to system ssh elsewhere.
-
-    ssh user@host
-
----