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>

30-shell-options

@@ -0,0 +1,21 @@
+# shellcheck shell=bash
+# Shell options: history, window size, and color support
+
+# History
+HISTCONTROL=ignoredups
+shopt -s histappend
+HISTSIZE=1000
+HISTFILESIZE=2000
+
+# Update LINES and COLUMNS after each command
+shopt -s checkwinsize
+
+# Enable color support for ls
+if command -v dircolors &>/dev/null; then
+    if [[ -r ~/.dircolors ]]; then
+        eval "$(dircolors -b ~/.dircolors)"
+    else
+        eval "$(dircolors -b)"
+    fi
+    alias ls='ls --color=auto'
+fi