fix(ci3): scope build-instance name by repo to stop cross-repo reaping (v5-next) (#23988)

## Problem

`ci3/bootstrap_ec2` terminates any existing instance that shares the
target `Name` tag before launching — this intentionally reaps orphans
left when a GA run is cancelled (e.g. by a new push) on the same ref.
But the name was `<ref>_<arch>[_<postfix>]` with **no repo component**,
so `aztec-packages` and `aztec-packages-private` — which build the same
tags/refs concurrently under the **same OIDC role** — computed identical
names and reaped each other's live instances.

### Observed incident

Nightly tag `v5.0.0-nightly.20260610` was built in **both** repos.
Instance `i-02e5d6a6c148ec726`
(`v5_0_0-nightly_20260610_arm64_a-release`) was launched by the private
repo's run at 03:06:01 UTC and **terminated at 03:13:12 UTC by the
public repo's run** for the same tag (its pre-launch reap step), ~7 min
in — failing the private build. CloudTrail confirms a
`TerminateInstances` from a different `ci3-<run_id>` session, not a spot
interruption.

## Fix

Prefix the instance name with the repo basename
(`${GITHUB_REPOSITORY##*/}`, defaulting to `aztec-packages` for local
runs):

- **Within a repo**, the key is unchanged in spirit
(`<repo>_<ref>_<arch>`) and stays stable across re-runs/new-pushes of
the same ref — so the intended orphan-on-cancel cleanup still works.
- **Across repos**, public → `aztec-packages_…` and private →
`aztec-packages-private_…`, so they no longer match and can't reap each
other.

`ci.sh`'s helper `instance_name` (used by the `shell`/`kill`/`get-ip`
dev commands) is kept in sync so it still resolves instances launched by
a CI run for the same repo.

### Notes

- The EC2 `Name` tag limit is 256 chars; the longest prefixed name is
~61 chars. The reap match uses the full `Name` tag, so the cosmetic
63-char `docker_hostname` truncation doesn't affect correctness.
- One-time transition: instances launched by the old (un-prefixed) code
won't be reaped by name-match from new runs; they fall back to the
shutdown timer / 1.5h reaper. Self-heals within a couple hours.
- This stops the *collision*. Whether public **and** private *should*
both build the same nightly tag (duplicated work) is a separate question
— happy to follow up if you want one gated off.

Author: ludamad

ci.sh

@@ -53,7 +53,11 @@ function print_usage {
 
 [ -n "$cmd" ] && shift
 
-instance_name=${INSTANCE_NAME:-$(echo -n "$BRANCH" | tr -c 'a-zA-Z0-9-' '_')_${arch}}
+# Keep this in sync with bootstrap_ec2's instance_name scheme (repo-scoped) so the
+# shell/kill/get-ip helpers find instances launched by a CI run for this repo.
+repo=${GITHUB_REPOSITORY##*/}
+repo=${repo:-aztec-packages}
+instance_name=${INSTANCE_NAME:-${repo}_$(echo -n "$BRANCH" | tr -c 'a-zA-Z0-9-' '_')_${arch}}
 [ -n "${INSTANCE_POSTFIX:-}" ] && instance_name+="_$INSTANCE_POSTFIX"
 
 function get_ip_for_instance {

ci3/bootstrap_ec2

@@ -65,11 +65,18 @@ if [[ "$(git fetch origin --negotiate-only --negotiation-tip="$current_commit")"
 fi
 
 # Our instance_name acts as a uniqueness key for the instance.
-# Instances are terminated if they exist with the same name.
+# Instances are terminated if they exist with the same name; this reaps orphans
+# left when a GA run is cancelled (e.g. by a new push) on the same ref.
+# Scope the key to the repo: aztec-packages and aztec-packages-private can build
+# the same tag/ref concurrently under the same role, and must not reap each
+# other's instances. The key stays stable across re-runs within a repo, so the
+# orphan cleanup still works.
+repo=${GITHUB_REPOSITORY##*/}
+repo=${repo:-aztec-packages}
 if [[ "$REF_NAME" =~ ^gh-readonly-queue/.*(pr-[0-9]+) ]]; then
-  instance_name="${BASH_REMATCH[1]}_$arch"
+  instance_name="${repo}_${BASH_REMATCH[1]}_$arch"
 else
-  instance_name=$(echo -n "$REF_NAME" | head -c 50 | tr -c 'a-zA-Z0-9-' '_')_$arch
+  instance_name="${repo}_$(echo -n "$REF_NAME" | head -c 50 | tr -c 'a-zA-Z0-9-' '_')_$arch"
 fi
 
 state_dir=$(mktemp -d /tmp/aws_request_instance.XXXXXX)