fix: auto-add install dir to PATH

[George-iam]

Summary

• Install script now automatically adds ~/.local/bin to PATH in the user's shell rc file
• Detects shell: zsh → .zshrc, bash → .bash_profile/.bashrc, fish → config.fish, other → .profile
• Idempotent: skips if PATH entry already exists in rc file or current PATH
• Also exports PATH in the running session so axme works immediately after install

Before

Add /Users/you/.local/bin to your PATH if it is not already present.

User had to manually edit rc file → axme: command not found.

After

Adding /Users/you/.local/bin to PATH in /Users/you/.zshrc...
Done. PATH updated for current and future sessions.

Test plan

• Fresh install on macOS zsh — verify .zshrc updated, axme version works
• Re-run install — verify no duplicate line added
• Install with PATH already containing ~/.local/bin — verify no rc file modification
• Install on Linux bash — verify .bashrc or .bash_profile updated