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

[charlielye]

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.