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/05-package-management.md

@@ -8,53 +8,4 @@ helpKeywords:
 - packages
 ---
 
-## pkg
 
-    Synopsis:  pkg [-h] [-i|-u] <package> [package...]
-    Installs or removes packages using the detected system package manager.
-    Supports: paru, yay, pacman, apt, dnf, zypper, yum, brew, pkg.
-
-      (no flag)    Auto mode: installs missing packages, removes installed ones
-      -i/--install  Force install
-      -u/--uninstall  Force uninstall
-
-    pkg firefox             # auto: install if missing, remove if present
-    pkg -i ripgrep fd       # force install
-    pkg -u cowsay           # force uninstall
-
-    The package-installed check uses the correct query for each PM:
-      pacman/paru/yay  pacman -Qi
-      apt              dpkg -s
-      dnf/zypper/yum   rpm -q
-      brew             brew list
-      pkg              pkg info
-
-## search
-
-    Synopsis:  search [args...]
-    Interactive AUR package search and install via paru or yay.
-    Arch Linux only.
-
-    search neovim
-
-## upgrade
-
-    Synopsis:  upgrade
-    Full system upgrade via paru -Syu --noconfirm or yay -Syu --noconfirm.
-    Arch Linux only.
-
-## cleanup
-
-    Synopsis:  cleanup
-    Lists and removes orphan packages via pacman, logging their names to
-    ~/.removed_orphans. Arch Linux only.
-
-## parur
-
-    Synopsis:  parur
-    Opens an fzf picker of all installed packages (with pacman -Qi previews),
-    then removes the selected packages via paru or yay. Arch Linux only.
-
-    parur
-
----