extension-bundles publish isn't wired to core release, so init --with can 404 with an unhelpful error in the gap #534

Split out of #532 for a standalone, independently-closable scope.

What's wrong

`urlcode init --with` auto-resolves `extension-bundles@v` with no fallback (that auto-resolve-with-no-fallback behavior is the intended hard rule from #526 -- not itself the bug here).

The bundle publish workflow (`.github/workflows/extension-bundles.yml`) only triggers on a manually pushed `extension-bundles@v*` tag or `workflow_dispatch`. It is not triggered by `release-core-dispatch.yml` or `candidate.yml`, so there is no guarantee the matching bundle release exists the moment a new core version ships. We confirmed this is a real, currently-manual-process-dependent window: 0.5.9's core release (`v0.5.9`, 17:31:26Z) and its matching bundle release (`extension-bundles@v0.5.9`, 17:40:48Z) landed ~9 minutes apart, not atomically.

If `init --with` runs during that window, `createGithubTransport`'s `release()` just throws `Could not fetch extension bundle release extension-bundles@v` -- a generic message that:

• doesn't explain there's a known timing gap between core and bundle releases,
• doesn't mention `--bundle-release ` as an escape hatch to pin an older, already-published, compatible release,
• is indistinguishable from an auth failure, a network failure, or a genuinely nonexistent core version.

Suggested fix

• Improve the error message on a failed release fetch to mention `--bundle-release ` as an explicit pin and suggest checking whether the bundle release for this core version has published yet.
• Separately, decide whether `extension-bundles.yml` should be triggered automatically as part of the core release pipeline (closing the window entirely), or whether the ~9-minute manual-coordination gap is an accepted cost worth just documenting.

Related

• init --with has no offline/local-bundle path and can 404 in the window before extension-bundles@v<core> exists #532 (superseded by this issue and the offline/local-bundle-transport issue split from it)