emacs: refresh archives and retry when a package install fails

[tlockney]

The failure

On m5air, adding magit produced:

⛔ Error (use-package): Failed to install magit:
   https://elpa.nongnu.org/nongnu/with-editor-3.4.9.tar: Not found

The cached archive index isn't wrong about which packages exist — it's wrong about their versions. GNU and nongnu ELPA keep only the current release of each package, so an index naming a superseded version resolves to a tarball the server has deleted. A 404, not a missing-package error.

Why nothing recovered

This is a regression from the startup work in #27. That change added:

(unless package-archive-contents
  (package-refresh-contents))

which refreshes only when the cache is empty — so after the first successful run on a machine, the index is never refreshed again and quietly rots.

use-package can't cover for it either. From use-package-ensure-elpa:

(if (assoc package package-archive-contents)
    (package-install package)      ; present: install, no refresh
  (package-refresh-contents)       ; absent: refresh, then install
  (package-install package))

magit was present in the stale cache, so it took the first branch and failed while resolving with-editor. Any newly added package hits this on any machine whose cache has aged; the only recovery was M-x package-refresh-contents by hand.

The fix

package-install now refreshes the archives and retries once on failure. It only runs on the error path, so a fresh cache costs nothing.

Verification

The advice was unit-tested across all three paths:

Case	Expected	Result
Install 404s once, then succeeds	refresh, retry, succeed	attempts=2 refreshed=t
Install fails persistently	error still surfaces, not swallowed	error surfaced after 2 attempts
Install succeeds first time	no refresh, no network	attempts=1 refreshed=nil

Full cold start against an empty init-directory: exit 0, no errors, 24 packages, magit-4.6.0 + with-editor-3.5.2.

What I could not reproduce end-to-end: I tried to recreate the exact 404 by corrupting a real cached nongnu index to name with-editor 3.5.999. package.el installed MELPA's date-versioned build (20260701.1252) instead, so it never hit nongnu's dead tarball — a difference in archive selection between that sandbox and m5air I did not chase further. The mechanism above is established by reading use-package-ensure-elpa and by the unit tests, not by a full reproduction.

Immediate unblock

On m5air, before this merges: M-x package-refresh-contents, then M-x package-install RET magit RET.