Integrate old-svovel config: paths, aliases, completions

Add ~/bin PATH, rbenv (disabled), debuginfod, sccache, LESS/ag/task
aliases, and cargo/rustup completions. Removes old-svovel/ directory.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

50-cargo-completion

@@ -0,0 +1,10 @@
+# shellcheck shell=bash
+# Enable bash completion for cargo (Rust package manager)
+if command -v rustc &>/dev/null; then
+    _cargo_comp="$(rustc --print sysroot)/etc/bash_completion.d/cargo"
+    if [[ -f "$_cargo_comp" ]]; then
+        # shellcheck source=/dev/null
+        . "$_cargo_comp"
+    fi
+    unset _cargo_comp
+fi