ci: integration artifact retention + toolchain bump-bot commit prefix

[Wikid82]

Two small CI changes, both follow-ups to #1306 (integration-test consolidation, squash-merged as 255a23df).

1. ci: bump integration image artifact retention to 3 days (f1697408)

Supervisor's non-blocking note on #1306. In .github/workflows/integration-tests.yml, the
charon-integration-image artifact retention goes 1 day → 3 days. The fan-out suite
jobs (cerberus / waf / rate-limit / crowdsec) no longer build the image themselves —
they docker load it from the build job's artifact — so a 1-day retention breaks GitHub's
"Re-run failed jobs" once a run is >24h old. 3 days covers realistic re-run windows at
negligible cost. No behavior change beyond retention.

2. ci: use chore prefix for the toolchain bump-bot commits and PRs (04233733)

Maintainer request. The open-bump-pr job in .github/workflows/toolchain-image.yml opened
its daily digest-refresh PR with a feat(security): title and commit message. feat: makes
release-please cut a minor release on every merge to main, so routine toolchain-digest
refreshes were bumping the version each time. Switched the bot PR title + commit-message to
chore(docker): refresh bundled proxy toolchain image (matching the sibling sync-pin-on-pr
style). The security label is retained; only the conventional-commit prefix changes.
docs/plans/current_spec.md updated so its example + commit-convention note match.

Affects future bot PRs only. Bot PR #1310 (already open with the old feat(security):
title) is being closed as a misfire.

Rebase note

Rebased onto main after #1309 (fix(ci): restore toolchain pin on main …) landed, so the
verify-toolchain-pin / merge-and-publish failures inherited from the previously-broken
main pin are resolved.

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