dx(skills): stop publishing dev test creds in /pr-test SKILL.md (#13201)

## Why

`.claude/skills/pr-test/SKILL.md` had the dev test account's email +
password hardcoded in **8 places** — the "Test user credentials" header,
two Redis rate-limit examples, three signup/login curl snippets, and the
agent-browser login flow. Because the repo is public, anyone on the
internet who read the SKILL could sign in to `dev-builder.agpt.co` as
that test user. The account was disabled on 2026-05-23 via
`banned_until=2126-04-29` once the leak surfaced.

## What

- Every literal `test@test.com` / `testtest123` occurrence replaced with
`$PR_TEST_USER_EMAIL` / `$PR_TEST_USER_PASSWORD` shell variables.
- New "Test user credentials" section mandates two acquisition paths, in
order:
  1. Read env vars when set (CI / preconfigured shells).
2. Otherwise ASK the user interactively at the start of the run; hold
creds only in shell vars for the duration; never echo or persist.
- Inline `: "${PR_TEST_USER_EMAIL:?...}"` / `:
"${PR_TEST_USER_PASSWORD:?...}"` guards force a hard failure if the
agent forgot to acquire creds.
- A "do NOT re-introduce a default" warning calls out the prior leak so
future SKILL edits don't reset progress.

## How

`jq -nc --arg e "$PR_TEST_USER_EMAIL" --arg p "$PR_TEST_USER_PASSWORD"
'{email:$e,password:$p}'` builds the signup payload safely (escapes
whatever the user pastes), then the existing curl/agent-browser flow
reads from the env. No behaviour change for local docker-compose runs
once the vars are set; dev-preview runs now require the agent to ask the
user for creds each session.

## Test plan

- [x] `grep -c "test@test.com\\|testtest123"
.claude/skills/pr-test/SKILL.md` → only the deliberate warning line
remains (`do NOT re-introduce a default`); zero functional references.
- [ ] Manual: re-run `/pr-test` against `dev-builder.agpt.co` with
`PR_TEST_USER_EMAIL` / `PR_TEST_USER_PASSWORD` set in the shell —
confirms the agent-browser login still works after the rewrite.
- [ ] Manual: unset both vars and re-run `/pr-test` — confirms the
guards trip and the agent prompts for creds instead of barreling on.