dotcli: Add dot kde save with schema-backed defaults and completion.

Introduces the dot kde subcommand family (fish dispatcher plus a Python
helper under commands/kde/), a flat identifier=value manifest, and
dot kde save in both its explicit-identifier and no-argument refresh
modes. Settings resolve to KDE's KConfigXT schema-backed mechanism via a
(rcfile -> [kcfg files]) mapping table auto-derived by scanning the
system schema directory (overridable via DOT_KDE_KCFG_DIR), plus a
hand-maintained exceptions list for schemas that only declare their
target rc file at runtime. Also wires tab-completion for dot kde save
identifiers, sourced live from that same mapping table.

.config/dot/tests/fixtures/kcfg/kwin.kcfg

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<kcfg xmlns="http://www.kde.org/standards/kcfg/1.0">
+  <kcfgfile arg="true"/>
+  <group name="Windows">
+    <entry name="BorderSize" type="String">
+      <default>Normal</default>
+    </entry>
+  </group>
+</kcfg>

.config/dot/tests/fixtures/kcfg/testrc.kcfg

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<kcfg xmlns="http://www.kde.org/standards/kcfg/1.0">
+  <kcfgfile name="testrc"/>
+  <group name="General">
+    <entry name="Greeting" type="String">
+      <default>Hello</default>
+    </entry>
+    <entry name="AliasedKey" key="RealKey" type="String">
+      <default>AliasDefault</default>
+    </entry>
+    <entry name="Some.Key With Spaces" type="String">
+      <default>SpacedDefault</default>
+    </entry>
+  </group>
+</kcfg>

.config/dot/tests/fixtures/kcfg/unmapped.kcfg

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<kcfg xmlns="http://www.kde.org/standards/kcfg/1.0">
+  <kcfgfile arg="true"/>
+  <group name="Whatever">
+    <entry name="Setting" type="String">
+      <default>Unreachable</default>
+    </entry>
+  </group>
+</kcfg>