Move inline .bashrc config into modular .bash.d/ scripts

Extracted history, shell options, dircolors, rbenv, ~/.local/bin PATH,
and Maestro PATH from ~/.bashrc into dedicated .bash.d/ files, reducing
.bashrc from 126 lines to a minimal 23-line loader.

New files:
- 10-local-path: ~/.local/bin via path_prepend
- 10-maestro-path: ~/.maestro/bin via path_append
- 20-rbenv: rbenv init with existence guard
- 30-shell-options: history, checkwinsize, dircolors

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

20-rbenv

@@ -0,0 +1,5 @@
+# shellcheck shell=bash
+# Initialize rbenv (Ruby version manager)
+if [[ -x "$HOME/.rbenv/bin/rbenv" ]]; then
+    eval "$("$HOME/.rbenv/bin/rbenv" init - --no-rehash bash)"
+fi