Speed up devcontainer creation by parallelizing tool installs and moving them into the prebuild

[arnaudlh]

Container creation was slow because post-create.sh ran three sequential installs (Checkov, PSRule.Rules.Azure, arm-ttk) on every create, outside the prebuild.

Changes

• devcontainer.json: switch postCreateCommand → onCreateCommand so the work is captured by Codespaces prebuilds and doesn't re-run on each create.
• post-create.sh: run the three independent installs in parallel; wall time ≈ slowest step instead of the sum.
• Faster flags: pwsh -NoProfile, Install-Module -AcceptLicense -SkipPublisherCheck; pip --no-compile --disable-pip-version-check; git clone --single-branch --filter=blob:none.
• Idempotent: skip steps when checkov is on PATH, PSRule.Rules.Azure is listed, arm-ttk.psd1 exists, or the profile import line is already present — re-runs (rebuild on existing volume) are near-instant.
• Failure attribution: each background job logs to its own file; on failure the script reports which install failed and its exit code.

pids[checkov]=$!
pids[psrule]=$!
pids[armttk]=$!
for name in "${!pids[@]}"; do
  if ! wait "${pids[$name]}"; then
    echo "==> '$name' install failed with exit code $?" >&2
    status=1
  fi
done

Fixes #35