bin proxies: *_PROXY_PORT parsed with bare parseInt — invalid values crash opaquely or bind the wrong port #483

Problem

All five local proxy scripts in bin/ parse their listen port with a bare parseInt(process.env.<VAR> || '<default>', 10) and pass the result straight to server.listen(...) with no validation:

• bin/codex-proxy.mjs:7 — CODEX_PROXY_PORT, default 18744
• bin/clef-proxy.mjs:7 — CLEF_PROXY_PORT, default 18742
• bin/agy-proxy.mjs:7 — AGY_PROXY_PORT, default 18743
• bin/antigravity-ls-proxy.mjs:7 — ANTIGRAVITY_LS_PROXY_PORT, default 18745
• bin/design-code-proxy.mjs:5,8,9 — DESIGN_CODE_PROXY_PORT (18746), ANTIGRAVITY_LS_PROXY_PORT (18745), CODEX_PROXY_PORT (18744)

Verified behaviour (Node, this repo's runtime)

parseInt is lenient, and server.listen is not a validator:

The worst case is the silent one: a mistyped or hex-spelled port leaves the proxy listening somewhere unexpected (or on an ephemeral port) while opencode's config points at the documented port — the failure surfaces far away as "proxy unreachable", with nothing in the proxy's own output naming the bad env value. The crash cases are only slightly better: ERR_SOCKET_BAD_PORT never says which env var was wrong or what value it carried.

This is the same failure class as the port-validation issues already fixed/filed for the sibling tools (openchamber schedule weekday parse, opencode restart-driver OC_PORT), but these five scripts are a separate copy that lives only in this repo.

Suggested fix

Parse each port through one strict helper (the scripts already share copy-pasted helpers, e.g. publishEndpoint — see the PR #438 review finding): accept only a decimal integer in 1–65535; on anything else, exit at startup with a message naming the env var and the offending value, e.g. CODEX_PROXY_PORT="0x4a38" is not a valid port (expected an integer 1-65535). Apply it at all seven parse sites in the five files.

Acceptance criteria

• Each of the five scripts rejects a non-numeric, fractional, suffixed, hex-spelled, zero, negative, or >65535 port value at startup with an error naming the env var and value, and does not bind any port.
• Valid values (including the defaults when the var is unset/empty) bind exactly the requested port, as today.
• A check/test exercises at least the abc, 0x…, and 99999 cases for one proxy and the shared helper (if extracted) is used by all seven sites.

Related but distinct: #434 (missing {env:…} fallback in opencode config), #435 (single-sourcing these ports), #437 (port discovery) — none of them validates the value itself.