#1045 repo upgrade fatally aborts on a yanked pinned dependency (HTTP 410) instead of forward-resolving

Description

swamp repo upgrade fatally aborts the entire migration when a pinned/target extension version has been yanked from the registry (HTTP 410), instead of forward-resolving to the available successor version or skipping it. The error guidance is also misleading.

Steps to reproduce

1. A repo that targets @atalanta/external-reviewer@2026.06.27.1, which was later yanked (successor @2026.06.28.1 is published and available).
2. swamp repo upgrade
3. Other extensions install fine ("Installed 2, 35 up to date"), then:

warning  1 extension(s) failed to install.
  "@atalanta/external-reviewer": UserError: Extension API error (HTTP 410): Version has been yanked
fatal    Error: Extension migration failed for 1 extension(s). Legacy files have been preserved.
         Re-run 'swamp repo upgrade' once the issue is resolved (usually registry access)

Expected vs actual

• Expected: a yanked pinned version resolves forward to the latest non-yanked version (here 2026.06.28.1) or is skipped with a warning; the upgrade completes.
• Actual: the whole upgrade exits fatal on one yanked (optional) extension. swamp extension info @atalanta/external-reviewer confirms 2026.06.28.1 is available, so forward-resolution was possible. Also, the hint "re-run once resolved (usually registry access)" is wrong for a yank — it is permanent, not a transient access issue; the real fix is a version bump.

Environment

swamp 20260708.202130.0-sha.3afd02a8; macOS arm64 (darwin 25.2.0)

Suggested fix

On a 410/yanked version during upgrade: forward-resolve to the latest non-yanked version, or skip+warn and continue the remaining extensions; and correct the guidance to advise a version bump rather than a retry.