fix: restore phone-home diagnostics in warm-pool download step

[kurok]

Summary

buildReusableUserData() (the reuse: stop / warm-pool cold-boot script) dropped the bootstrap ERR trap before the runner-download step and never re-established phone-home diagnostics inside that step's inner sudo -u runner shell — unlike buildUserData()'s plain-path equivalent, which does both. Functions and traps don't survive the sudo -u boundary, so any curl/sha256sum/tar failure inside the warm-pool RUNNER_DOWNLOAD heredoc was completely silent: no failed:downloading bootstrap tag, and mode: start just ran out the registration-timeout clock with nothing to show for it.

Fixes #61

Fix

• Re-include PHONE_HOME_HELPERS inside the RUNNER_DOWNLOAD heredoc.
• Set GH_RUNNER_STEP=downloading and re-arm trap 'gh_runner_phone_home "failed:${GH_RUNNER_STEP}"' ERR inside that heredoc, before the download/verify sequence — mirroring buildUserData()'s RUNNER_BOOTSTRAP heredoc line for line.
• Also tightened ordering in the plain path's configuring phase (GH_RUNNER_STEP=configuring + trap now armed immediately after PHONE_HOME_HELPERS, ahead of the IMDS/config.sh prep lines) for consistency.

Tests added

New regression tests in tests/warmpool.test.js, scoped to the RUNNER_DOWNLOAD heredoc body specifically (via a helper that extracts just that heredoc's contents) so they can't be satisfied by matches elsewhere in the script — e.g. the outer root shell's preparing trap or the register script's configuring trap:

• re-includes PHONE_HOME_HELPERS inside the RUNNER_DOWNLOAD heredoc (regression, #61)
• arms an ERR trap for the downloading phase inside the RUNNER_DOWNLOAD heredoc (regression, #61) — also asserts ordering: GH_RUNNER_STEP=downloading and the trap must be set before the curl -fsSLo download call.

Verification

• npm test — 17 suites / 226 tests, all passing.
• npm run lint — clean, no findings.
• npm run package — rebuilt dist/index.js; git status --porcelain -- dist/ confirmed the working-tree dist/ already matched a fresh build (no drift beyond the intended 8-line diff mirroring the src/aws.js change). Only dist/index.js changed; the code-split dist/*.index.js chunk files are untouched by this fix.

Review findings

A code review pass was run against this change; no findings were reported (confirmedCount: 0, no fixes needed/applied).