docs(functions): merge remaining manual-only facts into headers

Word-level diff of every manual entry against its generated counterpart
surfaced 546 tokens present in the manual and absent from the header —
losses reconcile.py missed, because it compared description length and
these headers are longer overall thanks to ARGUMENTS/RETURNS.

Merged the substantive ones (546 -> 202 residual tokens, the remainder
being synonym drift). Notable real fixes:

- git-clean: -f/--force was missing from ARGUMENTS entirely
- pkg: per-package-manager query table
- qc: cli-role rationale, role paths, --role passthrough
- config-help: site URL, xdg-open, man page path, case-insensitivity
- agents-init: idempotency, .gitignore paths, upstream pull, wrapper callers
- smart_exit: exit-builtin wiring note, $SCROLLBACK_HISTORY_DIR

The manual's claim that claude/agy pass `agents-init --agents` is stale;
both pass `--quiet` (full setup). Header wins, manual dropped.

functions/du.fish

@@ -8,14 +8,14 @@
 #   du [--disk|--dir|--dua] [args...]
 #
 # DESCRIPTION
-#   Smart disk-usage wrapper that routes to duf (disk overview), dust (directory
-#   tree), or dua based on context or explicit flags. Falls back to system du
-#   when the preferred tool is not installed.
+#   Smart disk-usage dispatcher. Without flags, routes to the most appropriate
+#   tool by context; explicit flags force one. Falls back to system du when the
+#   preferred tool is not installed.
 #
 # ARGUMENTS
-#   --disk   Force duf for disk-level overview
-#   --dir    Force dust for directory-level breakdown
-#   --dua    Force dua interactive mode
+#   --disk   Force duf  (disk-level free/used overview)
+#   --dir    Force dust (per-directory tree breakdown)
+#   --dua    Force dua  (fast interactive space analyzer)
 #   args...  Files/directories or flags forwarded to the selected tool
 #
 # EXAMPLE