revert: remove Packer snapshot pipeline (#2205)

DO snapshots are private and account-scoped — users on different
accounts cannot see snapshots built by the CI token. Docker images
are the better approach for cross-account pre-built agents.

Removes: packer/, packer-snapshots workflow, snapshot lookup code,
and snapshot test. Reverts DO CLI to plain cloud-init flow.

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

packer/agents.json

@@ -1,45 +0,0 @@
-{
-  "claude": {
-    "tier": "minimal",
-    "install": [
-      "curl -fsSL https://claude.ai/install.sh | bash || npm install -g @anthropic-ai/claude-code"
-    ]
-  },
-  "codex": {
-    "tier": "node",
-    "install": [
-      "npm install -g @openai/codex"
-    ]
-  },
-  "openclaw": {
-    "tier": "full",
-    "install": [
-      "npm install -g openclaw"
-    ]
-  },
-  "opencode": {
-    "tier": "minimal",
-    "install": [
-      "curl -fsSL https://opencode.ai/install | bash"
-    ]
-  },
-  "kilocode": {
-    "tier": "node",
-    "install": [
-      "npm install -g @kilocode/cli"
-    ]
-  },
-  "zeroclaw": {
-    "tier": "minimal",
-    "install": [
-      "fallocate -l 4G /swapfile && chmod 600 /swapfile && mkswap /swapfile && swapon /swapfile",
-      "curl -LsSf https://raw.githubusercontent.com/zeroclaw-labs/zeroclaw/a117be64fdaa31779204beadf2942c8aef57d0e5/scripts/bootstrap.sh | bash -s -- --install-rust --install-system-deps --prefer-prebuilt"
-    ]
-  },
-  "hermes": {
-    "tier": "minimal",
-    "install": [
-      "curl -fsSL https://raw.githubusercontent.com/NousResearch/hermes-agent/main/scripts/install.sh | bash"
-    ]
-  }
-}