Refactor bootstrap.sh: eliminate duplication, derive environments dynamically

[Copilot]

Addresses PR review feedback from thread #3769662762. Package parsing was duplicated in two locations, and environment lists were hardcoded in three places (help text, interactive menu, "all" loop), creating drift risk when adding new environments.

Extracted package parsing helper

• parse_packages() function now used in both setup_apt_environment() and batch install path
• Single source of truth for comment/whitespace handling

Dynamic environment discovery

• get_available_environments() scans apt/ directory at runtime
• Interactive menu, help text, and "all" loop now derive from same source
• Adding new environment directory automatically makes it available

Input validation

• Numeric validation for user choice
• Empty environment name filtering
• Safe undefined variable handling (${all_option:-})
• Graceful empty directory list handling

Example flow

# Before: hardcoded array, duplicated parsing
envs=("common" "test" "docker" ...)
pkgs=$(sed 's/#.*//' "apt/$env/packages.txt" | xargs)

# After: discovered dynamically, centralized parsing
envs=($(get_available_environments))
pkgs=$(parse_packages "apt/$env/packages.txt")

Preserves all batch installation optimizations. Zero functional changes to package installation behavior.

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