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/04-git-and-version-control.md

@@ -7,84 +7,4 @@ helpKeywords:
 - git
 ---
 
-## auto-pull
 
-    Synopsis:  auto-pull [list]
-               auto-pull add [PATH]
-               auto-pull remove <NAME|PATH>
-               auto-pull status
-
-    Manages the registry of repositories that are background fast-forwarded
-    when you enter them (see "Auto-pull fast-forward" under the C2 component
-    reference). The fish-config repo is always covered as a baseline. The
-    registry is machine-local at `$__fish_user_dots_path/auto-pull.list` (defaults
-    to `~/.config/.user-dots/fish/auto-pull.list`), one absolute path per line,
-    and is never committed. Registry management works
-    even when C2 auto-execution is disabled; only the background sync is gated.
-
-      list               Show registered repos (default)
-      add [PATH]         Register PATH's git root (default: current repo)
-      remove <NAME|PATH> Unregister by basename or exact path
-      status             Show enabled/disabled state, repo count, list path
-
-    cd ~/src/qmk_firmware; and auto-pull add
-    auto-pull add ~/work/api
-    auto-pull list
-    auto-pull remove qmk_firmware
-
-## branch
-
-    Synopsis:  branch <branch_name>
-    Switches to a local branch, or creates it if it does not exist.
-
-    branch feature/new-ui
-
-## gi
-
-    Synopsis:  gi [-h] [-b] [-p] [-s] [-l] [targets...]
-    Generates .gitignore content from the gitignore.io API with MD5-based
-    deduplication (patterns already present are not re-appended).
-
-      -b/--boilerplate  Append generic boilerplate first
-      -p/--prompt       Prompt interactively for targets
-      -s/--stdout       Print to stdout instead of appending to .gitignore
-      -l/--list         List all available targets
-      targets           Comma-separated or space-separated target names
-
-    gi python,venv
-    gi -b -p
-    gi -s node > .gitignore
-
-## git-clean
-
-    Synopsis:  git-clean [-f]
-    Fetches and prunes the remote, fast-forwards the current branch, then
-    deletes local branches whose remote tracking branch has been deleted.
-    Switches to main/master automatically if the current branch is orphaned.
-
-      -f/--force  Force-delete unmerged branches too
-
-    git-clean
-    git-clean --force
-
-## gitup
-
-    Synopsis:  gitup [args...]
-    Fetches updates from the remote and shows git status. Extra args are
-    forwarded to git fetch.
-
-    gitup
-    gitup --all
-
-## gitui
-
-    Synopsis:  gitui [args...]
-    Launches gitui with the Catppuccin Frappe theme pre-applied.
-
-## hist
-
-    Synopsis:  hist
-    Searches shell history with fzf, inserts the selection into the command
-    line, and copies it to the clipboard via wl-copy.
-
----