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>

packer/agents.json

@@ -2,7 +2,7 @@
   "claude": {
     "tier": "minimal",
     "install": [
-      "curl -fsSL https://claude.ai/install.sh | bash || mkdir -p ~/.npm-global/bin && npm install -g --prefix ~/.npm-global @anthropic-ai/claude-code"
+      "curl -fsSL https://claude.ai/install.sh | bash || [ -f /root/.local/bin/claude ]"
     ]
   },
   "codex": {