Switch install script default from /usr/local/bin to ~/.local/bin

[BRO3886]

Problem

The install script (scripts/install.sh, served at rem.sidv.dev/install) defaults INSTALL_DIR to /usr/local/bin:

INSTALL_DIR="${INSTALL_DIR:-/usr/local/bin}"

On modern macOS this requires sudo for almost everyone because:

1. On Apple Silicon, Homebrew owns /opt/homebrew/bin, and /usr/local/bin is owned by root:wheel with no group write — so anyone without Intel-Homebrew doesn't have write access
2. First-time installs prompt for a sudo password, which is a trust hit for a one-liner curl | bash install. Users correctly hesitate when a pipe-to-bash demands sudo
3. There's no good reason to need root for a single-user CLI tool

Proposed change

Switch the default install location to $HOME/.local/bin (or $HOME/.bin if preferred), following the XDG-ish convention that rustup, uv, mise, bun, deno, and most other modern single-binary installers now use.

INSTALL_DIR="${INSTALL_DIR:-$HOME/.local/bin}"

The install script should:

1. Create $HOME/.local/bin if it doesn't exist (mkdir -p)
2. Check if it's on PATH and, if not, print a clear one-liner for the user's shell to add it (detect $SHELL → append to .zshrc / .bashrc / .config/fish/config.fish)
3. Still honor INSTALL_DIR=... env var for users who want to override (existing behavior)
4. Keep the /usr/local/bin path working as a fallback if the user explicitly sets it — don't break existing installs

Why .local/bin over .bin

• ~/.local/bin is the XDG Base Directory convention and ships on PATH by default on most modern Linux distros (and Homebrew on macOS Sonoma+ adds it to PATH automatically if it exists)
• ~/.bin is older, non-standard, and more likely to collide with dotfile managers or be overwritten
• rustup, uv, mise, bun, cargo-binstall all use .local/bin or .cargo/bin / .bun/bin-style tool-specific dirs
• Users installing multiple tools via curl | bash scripts end up with everything in .local/bin anyway

Migration for existing users

• Existing installs in /usr/local/bin/rem keep working as long as /usr/local/bin is on PATH (it is by default)
• When an existing user re-runs the install script, it will install into .local/bin instead and then $PATH lookups will prefer whichever comes first
• Print a notice if /usr/local/bin/rem exists and is different from the new install, suggesting sudo rm /usr/local/bin/rem to avoid confusion

Documentation updates needed

• website/static/install (served script) — update the default and the path-check messaging
• website/content/docs/getting-started.md — update any references to the install location
• skills/rem-cli/SKILL.md — if it mentions the install location
• README.md — if it mentions it

Risk

Low. The change is backwards compatible (env var override still works), and the worst case is a user has to export PATH="$HOME/.local/bin:$PATH" once. Better than the current default which needs sudo.

Opening this as a follow-up to the v0.10.0 release. Not urgent enough to block anything but worth doing before the install footprint grows further.