feat: install brev CLI to ~/.local/bin without requiring sudo

[devin-ai-integration]

Summary

Changes the brev CLI install scripts to install the binary to ~/.local/bin instead of /usr/local/bin, removing the requirement for sudo during installation and upgrades.

Install scripts (bin/install-latest.sh, bin/install-latest-linux.sh):

• Install target changed from /usr/local/bin/brev (via sudo) → ~/.local/bin/brev (no sudo)
• Creates ~/.local/bin if it doesn't exist
• Prints a PATH warning with copy-paste fix if ~/.local/bin is not in $PATH

Upgrade command (pkg/cmd/upgrade/):

• Removed sudo.Gater dependency from the direct-install upgrade path
• Replaced sudo gate with a simple confirmation prompt (matching the Homebrew upgrade UX)
• Updated install path message to ~/.local/bin/brev

README: Removed sudo prefix from Linux and WSL install commands.

Review & Testing Checklist for Human

• Upgrade path for existing users: If a user previously installed to /usr/local/bin/brev via sudo, running brev upgrade will now place the new binary in ~/.local/bin/brev — but the old binary in /usr/local/bin will likely shadow it on PATH. Consider whether the install script should detect and remove/warn about a stale /usr/local/bin/brev.
• Shell-specific PATH guidance: The PATH warning only suggests appending to ~/.bashrc. Zsh users (default on macOS) would need ~/.zshrc instead. Verify this is acceptable for the Linux-only install script, or consider detecting $SHELL.
• CI / root environments: Confirm that installing to $HOME/.local/bin is acceptable for automated/CI use cases where the old sudo-based install was used.
• Manual test: Run bash -c "$(curl -fsSL .../install-latest.sh)" on a clean Linux environment and verify the binary lands in ~/.local/bin, the PATH warning appears if needed, and brev --version works after adding to PATH.

Notes

• The sudo package and its usage in register/deregister commands (for netbird, systemctl) are intentionally left unchanged — those operations genuinely require elevated privileges.
• Existing unit tests for the upgrade command all pass with the gater field removed.
• golangci-lint could not run due to a pre-existing Go version mismatch in the environment (lint built with Go 1.23, project targets 1.24); this is not related to this change.

Link to Devin session: https://app.devin.ai/sessions/0d03f0bca5b54573b7dd0109a22afe65
Requested by: @theFong

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring