fix: local-wp --help silently exits 1 with zero output

[Copilot]

local-wp --help (and every invocation) silently crashed under set -e because detect_php_bin returned exit code 1 when no macOS Local by Flywheel PHP directories exist — killing the script before any output was produced.

Root cause

set -euo pipefail
# ...
PHP_BIN="${PHP_BIN:-$(detect_php_bin)}"  # detect_php_bin returns 1 → set -e kills script

Any invocation on a system without Local installed (Linux, CI, fresh macOS) hit this on the way to the argument parser — including --help.

Changes (dist/bin/local-wp)

• --help/-h handled before PHP detection — early arg scan at the top exits cleanly before detect_php_bin is ever called
• detect_php_bin always returns 0 — explicit return 0 on every path; empty stdout signals "not found"
• || true guard on PHP_BIN assignment — belt-and-suspenders against any future regression:

  PHP_BIN="${PHP_BIN:-$(detect_php_bin || true)}"

• Graceful fallback to system PHP — when Local PHP is absent, warns and continues rather than dying silently; errors with actionable message if no PHP at all

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