Brew: Prompt users to update when new version is released

[computermode]

https://entire.io/gh/entireio/cli/trails/246

When there is a new version of the CLI available, offer for the user to brew upgrade to increase discoverability and promote adoption of newer releases.

Inspired by @pfleidi's discovery of Codex's auto-update prompt:

Tested via a local test binary:

TEST_ROOT=/private/tmp/entire-pr1057-test
FAKE_HOME="$TEST_ROOT/home"
FAKE_BIN="$TEST_ROOT/bin"
FAKE_CELLAR="$TEST_ROOT/fakebrew/Cellar/entire/0.0.1/bin"

mkdir -p "$FAKE_HOME" "$FAKE_BIN" "$FAKE_CELLAR"

cat > "$FAKE_BIN/brew" <<'EOF'
#!/bin/sh
echo "[fake brew] brew $*"
exit 0
EOF
chmod +x "$FAKE_BIN/brew"

go build \
  -ldflags "-X github.com/entireio/cli/cmd/entire/cli/versioninfo.Version=0.0.1" \
  -o "$FAKE_CELLAR/entire" \
  ./cmd/entire

env -u ENTIRE_NO_AUTO_UPDATE -u CI -u ENTIRE_TEST_TTY \
  HOME="$FAKE_HOME" \
  PATH="$FAKE_BIN:$PATH" \
  "$FAKE_CELLAR/entire" version

Update available! 0.0.1 -> 0.5.6
Release notes: https://github.com/entireio/cli/releases/tag/v0.5.6
1. Update now (runs `brew upgrade entire`)
2. Skip
3. Skip until next version
Press enter to continue
Choose an option [1]: 1

Updating Entire CLI: brew upgrade entire
[fake brew] brew upgrade entire
Update complete. Re-run entire to use the new version.

➜  cli git:(brew-update-msg) ✗ command -v entire
/private/tmp/entire-pr1057-test/fakebrew/Cellar/entire/0.0.1/bin/entire -v
/Users/ninawork/entire/devenv/cli/entire
Entire CLI 0.0.1 (unknown)
Go version: go1.26.2
OS/Arch: darwin/arm64

---

Note

Medium Risk
Moderate behavior change in the CLI update flow: introduces a new interactive prompt path for Homebrew installs and persists a new SkippedVersion field to the version-check cache, which could affect when users are notified or prompted to update.

Overview
Adds a Homebrew-specific auto-update prompt that shows release notes and offers Update now / Skip / Skip until next version, running brew upgrade entire when accepted.

Extends the version-check cache with SkippedVersion and wires CheckAndNotify to suppress notifications for the skipped latest tag until a newer release is available. Also standardizes update messaging ("Update available!" + release notes link) and switches Homebrew update commands from brew upgrade --cask ... to brew upgrade ..., with expanded test coverage for the new prompt and caching behavior.

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