feat: per shell switching

[Rijoanul-Shanto]

What

v2.5.0 > per-shell switching

• phpvm shell pins PHP for the current terminal only via a php shim on PATH, no sudo, no effect on other shells
• phpvm local writes .php-version (no sudo); phpvm global moves the update-alternatives symlink (sudo). --set / --set-project kept as aliases
• Three-layer resolution: shell pin (PHPVM_SHELL_VERSION) > project (PHPVM_AUTO_VERSION from cd-hook) > global symlink
• TUI pins the current shell on Enter when launched through the wrapper; g for global, p for project
• phpvm --current reports all three layers plus the effective version - --doctor gains a per-shell switching section
• Shell hooks (bash/zsh/fish) rewritten: shim prepended to PATH, phpvm() wrapper routes shell and bare TUI through eval/| source
• install.sh installs the shim, enables the hook by default

Why

• Per-shell switching is the standard model (rbenv, pyenv, asdf) and the most requested missing feature: two terminals on two versions at once, no sudo on every switch, and a shell pin that survives cd into a project with a different .php-version

Tested

• bash phpvm.sh runs without errors
• shellcheck phpvm.sh passes (CI gate at -S warning)
• 33/33 tests pass in tests/test_cli.sh, including new coverage for --current three-layer output, local version normalization (php8.1 and 8.2.0), and shim fallback/pin behaviour
• CI matrix (Ubuntu 20.04 / 22.04 / 24.04) all 7 jobs green
• phpvm install tested in a clean Docker container; debconf hang confirmed fixed with DEBIAN_FRONTEND=noninteractive
• TUI wrapper mode tested with tmux pty-driving in a container (Enter pins shell, g switches global, q quits)
• Tested on Bash version: 5.1.16 (Ubuntu 22.04)