feat(ci3): give single-instance PR runs a parent dashboard log

[charlielye]

Problem

Merge-queue runs show a top-level "parent log" on the CI dashboard that includes the spot/instance request (which instance type was created, spot vs on-demand). Standard PR runs don't — to see what instance a PR run got, you have to leave the dashboard and dig into the GitHub Actions console.

Cause

The runner-side orchestration output (the Requesting spot fleet… line from aws_request_instance_type) is printed on the GA runner, before the remote build streams to its per-instance CI_LOG_ID log. Where that runner-side output lands depends on the path in ci.sh:

• Merge-queue goes through multi_job_run, which pipes everything into a parent dashboard log: parallel … 'run …' | DUP=1 cache_log "CI run" $RUN_ID. Each run() wraps bootstrap_ec2 with PARENT_LOG_ID=$RUN_ID, so the instance request lands in the parent log and the build log links underneath it.
• PR modes called bootstrap_ec2 directly — no cache_log, no parent log — so the instance request only reached the GA console.

Change

Route the PR-facing single-instance modes through the same multi_job_run path (with a single job), so they get an identical "CI run" $RUN_ID parent log with the instance request visible and the build log linked beneath it:

• fast / docs / barretenberg / barretenberg-full
• full / full-no-test-cache
• chonk-input-update

The job id is kept as x-$cmd, so the ci/<job> GitHub commit-status name is unchanged (no impact on required checks). socket-fix (which takes extra args and is an interactive debug mode) keeps its raw, un-denoised output but now pipes through cache_log so it also gets a parent log.

Behavior notes

• PR-run GA console output is now denoised (condensed progress) for the converted modes, matching merge-queue; the full log lives in the dashboard parent log.
• Instances for these modes now carry an INSTANCE_POSTFIX equal to the job id (e.g. x-fast), so the EC2 Name tag becomes <ref>_amd64_x-fast. Same-mode re-runs still dedupe correctly.

Validation

This is a structural reuse of the already-proven merge-queue path (multi_job_run), and bash -n ci.sh passes. It can't be exercised locally (needs the GA + AWS orchestration), but this PR's own CI run is the test: the fast job should now produce a CI run parent log on the dashboard showing the instance request, reachable without opening GitHub Actions.