Improve CLAUDE.md docs and add bash style rules

Expand CLAUDE.md with detailed helper function reference,
environment variables section, and commit conventions.
Add .claude/rules/bash-style.md for bash coding standards.

.claude/rules/bash-style.md

@@ -0,0 +1,64 @@
+---
+paths:
+  - "**/*.sh"
+  - "bashrc"
+  - "bin/*"
+---
+
+# Bash Readability Style
+
+The primary goal is code that is **easy to understand at a glance**. Write bash that reads naturally, follows established conventions, and doesn't require the reader to puzzle over clever tricks. Favor clarity and well-known idioms over brevity.
+
+Complements CLAUDE.md. Does not repeat rules already defined there.
+
+## Formatting
+
+- 2-space indentation, no tabs
+- `then`/`do` on same line: `if cond; then` / `for x in list; do`
+- Blank lines between logical sections; no blanks between tightly coupled statements
+- Long pipe chains: one command per line with leading `|`
+
+## Naming
+
+- `UPPER_CASE` — exported env vars and constants
+- `snake_case` — local variables and function names
+- `verb_noun` pattern for functions: `set_ps1_prompt`, `docker_aws_login`
+- Meaningful names, no single letters except loop counters (`i`, `f`)
+
+## Functions
+
+```bash
+function do_thing() {
+  local name="$1"
+  local count="${2:-0}"
+  ...
+}
+```
+
+- Always `function name() {` declaration style
+- Always `local` for function variables, assigned on declaration line
+- Name parameters immediately: `local filename="$1"` — no raw `$1` in logic
+- Default values via `${VAR:-default}`
+- Guard clauses with early `return` over deep nesting
+- Keep functions focused — single responsibility
+
+## Syntax Preferences
+
+- `[[ ]]` over `[ ]` for conditionals
+- `(( ))` for arithmetic comparisons
+- `$(command)` for substitution, never backticks
+- Double quotes around expansions: `"$var"`, `"${array[@]}"`
+- Single quotes for literals that must not expand
+- `printf` over `echo` when output contains escapes or format strings
+
+## Commands
+
+- Long options for clarity: `--recursive` not `-r` (common test flags `-f`, `-d`, `-e` are fine)
+- Redirect stderr: `> /dev/null 2>&1`
+- `# shellcheck shell=bash` at top of sourced (non-executable) files
+
+## Comments
+
+- Minimal — explain *why*, not *what*
+- URL reference when pattern comes from external source
+- No banner/divider comments