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/03-editors-and-viewers.md

@@ -7,65 +7,4 @@ helpKeywords:
 - editors
 ---
 
-## edit
 
-    Synopsis:  edit [-V|-t] [-e EDITOR] [-c] [-x TEXT] [-n] [-v|-s] [FILE...]
-
-    Opens files in a text editor, choosing a terminal or GUI editor and
-    resolving a rich chain of fallbacks. With no --visual/--terminal flag the
-    mode is auto-detected: interactive terminals use the terminal editor
-    ($EDITOR), while detached invocations (e.g. desktop shortcuts) use the GUI
-    editor ($VISUAL). Clipboard contents and literal strings can be opened as
-    throwaway temp files. Editor output is suppressed unless --verbose.
-
-    GUI fallback chain:      zed → antigravity-ide → code → kate → kwrite →
-                             gnome-text-editor → gedit
-    Terminal fallback chain: nvim → vim → micro → nano → vi
-
-    Options:
-      -V, --visual      Force the GUI editor ($VISUAL or fallbacks)
-      -t, --terminal    Force the terminal editor ($EDITOR or fallbacks)
-      -e, --editor=X    Use a specific editor binary X
-      -c, --clipboard   Open the clipboard contents (as a temp file)
-      -x, --text=STR    Open STR as the contents of a new temp file
-      -n, --new         Force a new window/instance (best-effort)
-      -v, --verbose     Print the launch command and editor output
-      -s, --silent      Suppress all output, including the editor's
-      -h, --help        Show this help message
-
-    edit ~/.config/fish/config.fish
-    edit --visual notes.txt
-    edit --terminal --new todo.md
-    edit --editor=code --clipboard
-    edit --text="hello world"
-
-## fc
-
-    Synopsis:  fc [command_prefix]
-    Edit the last shell command (or one matching a prefix) in $EDITOR,
-    then execute the result. Bash-style fc behaviour.
-
-    fc
-    fc git
-
-## less
-
-    Synopsis:  less [args...]
-    Pager wrapper with fallback chain: $PAGER -> ov -> less -> more -> cat.
-
-    less /var/log/syslog
-
-## rawfish
-
-    Synopsis:  rawfish [args...]
-    Launches Fish with NO_TMUX=1, bypassing any tmux auto-attach logic.
-    Useful when you need a clean shell without session management.
-
-## view
-
-    Synopsis:  view [args...]
-    Opens files in nvim read-only mode (-R). Falls back to less.
-
-    view /etc/fstab
-
----