feat(completions): unify cd/z completions across CWD, CDPATH, and zoxide

Previous to this commit, tab completion and auto-suggestions for cd/z
were inconsistent — only one or two of: CWD, CDPATH, and zoxide frecency
results would work at a time, with different sourced matches shown in
tab completion, shown in auto-suggest, and execution after pressing
<Enter>.

- Add _zoxide_z_complete in functions/zoxide.fish that merges all three
  sources into a single completion list (CWD via __fish_complete_cd,
  CDPATH via __fish_complete_directories, zoxide via query -l capped at
25)
- Wire the new completer to both z and cd via complete directives in
  conf.d/zoxide.fish, replacing the previous incomplete approach
- Add completions/zoxide.fish for full tab completion of the zoxide CLI
  itself (add, query, remove, import, init subcommands)
- Update README to document the unified completion behavior and fix
  structural issues in Personalization/Attribution/Dependencies sections

functions/zoxide.fish

@@ -0,0 +1,45 @@
+# Adapted from icezyclon/zoxide.fish (MIT)
+# Heavily customized for Fish 4.x compatibility and performance
+
+function _zoxide_z_complete -d "Complete directory first or zoxide queries otherwise" --argument-names comp desc
+    # comp is the currently completing token
+    if not set -q comp[1]
+        set comp (commandline -ct)
+    end
+    # cmd are all tokens including the current one except the command
+    set -l cmd (commandline -opc) $comp
+    set -e cmd[1]
+
+    # 1. Get standard completions (CWD, CDPATH, etc.)
+    # We call the underlying functions directly to avoid recursion.
+    if test (count $cmd) -le 1
+        # CDPATH results
+        __fish_complete_cd
+        # Local directory results (CWD)
+        __fish_complete_directories "$comp" ""
+    end
+
+    # 2. Get zoxide results
+    # Cap results to 25 to avoid overwhelming the completion engine
+    set -l zresults (zoxide query -l $cmd | head -n 25)
+    for res in $zresults
+        set -l bname (basename $res)
+        # If the basename matches the prefix, show it as a short name.
+        if string match -qi "$comp*" -- $bname
+            printf "%s/\tzoxide: %s\n" $bname $res
+        end
+        # Also provide the absolute path. Fish will filter it if it doesn't match.
+        printf "%s/\tzoxide\n" $res
+    end
+end
+
+function _zoxide_equals_first_token -a check -d "Test if first non-switch token equals given one"
+    set -l tokens (commandline -co)
+    set -e tokens[1]
+    set -l tokens (string replace -r --filter '^([^-].*)' '$1' -- $tokens)
+    if set -q tokens[1]
+        test $tokens[1] = $check
+    else
+        return 1
+    end
+end