fix: snapshot cleanup + claude install (name-prefix filter) (#2273)

* fix: claude snapshot build — remove npm fallback from install command

The native install (curl | bash) succeeds but exits non-zero due to a
PATH warning. The || fallback then tries `npm install` which doesn't
exist on the "minimal" tier → exit 127.

Fix: replace npm fallback with binary existence check (same pattern
as hermes agent). If install exits non-zero but ~/.local/bin/claude
exists, the build succeeds.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: snapshot cleanup and lookup — use name prefix instead of tags

DO Packer builder `tags` only apply to the temporary build droplet,
not the resulting snapshot image. Both the workflow cleanup step and
the CLI's findSpawnSnapshot() were querying by `tag_name` which
returned nothing — old snapshots piled up and the CLI couldn't find
existing snapshots.

Fix: filter by snapshot name prefix (`spawn-{agent}-`) instead of
tags, in both the workflow and the CLI. Remove misleading `tags`
from the Packer template. Add test cases for name-prefix filtering.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

packages/cli/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@openrouter/spawn",
-  "version": "0.15.6",
+  "version": "0.15.7",
   "type": "module",
   "bin": {
     "spawn": "cli.js"