Default install to ~/.local/bin to avoid requiring sudo

[albertodebortoli]

Summary

install.sh / uninstall.sh defaulted INSTALL_DIR to /usr/local/bin, which is root-owned on stock macOS. Even though the scripts already had sudo_if_install_dir_not_writeable() to skip sudo when possible, that helper never actually got a chance to skip it because the default target directory itself required elevated privileges — every fresh install prompted for a password.

This PR switches the default to the XDG-style ~/.local/bin (a common no-sudo convention, used by tools like pipx, uv, rustup), while keeping /usr/local/bin fully supported as an opt-in via INSTALL_DIR=/usr/local/bin ./install.sh.

• install.sh
  • INSTALL_DIR now defaults to $HOME/.local/bin instead of /usr/local/bin.
  • New "PATH setup" step: if $INSTALL_DIR isn't already on PATH for the current session, appends an export PATH=... line to the detected shell RC file (.bashrc/.zshrc), idempotently (skips if the exact line is already present), and exports it into the current process too. If the shell can't be detected, prints a manual instruction instead of failing.
  • Deduplicated shell RC detection: previously the script detected $SHELL → RC file in two separate, identical case blocks (once for the new PATH step, once for the final "how to start using Luca" message). Consolidated into a single SHELL_PROFILE variable computed once and reused by both.
• uninstall.sh
  • INSTALL_DIR default updated to match.
  • Removes the PATH export line it added, alongside the existing shell-hook cleanup.
• README.md
  • Updated the mermaid diagrams (install flow, uninstall flow, directory structure, sequence diagram) to reflect ~/.local/bin as the default and the new PATH-bootstrap step.

Notes

• If PATH already contains $INSTALL_DIR in the running shell (e.g. because another tool like uv/rustup already sources an env file that adds ~/.local/bin), the script correctly no-ops on the RC file — it doesn't need to duplicate PATH setup that's already handled elsewhere.
• Existing installs at /usr/local/bin (from before this change) are unaffected; they just won't be found unless INSTALL_DIR=/usr/local/bin is set, or the user re-runs uninstall with that override before switching to the new default location.

Test plan

• bash -n install.sh / bash -n uninstall.sh — syntax check
• bats tests/*.bats — full suite (60/60) passes unchanged, since tests already override INSTALL_DIR explicitly
• Manually verified on a real machine: fresh terminal picks up luca from ~/.local/bin without sudo; confirmed the "already on PATH" fast path correctly skips redundant RC edits when another tool (uv's ~/.local/bin/env) already manages PATH