docs: AMI build + boot-mode + cleanup playbook (infra/ami/README.md)

Covers: prerequisites (Packer + AWS profile), `packer init` + `packer
build` invocation, sharing the AMI cross-account via `ami_users`, the
`LITELLM_AGENT_MODE` boot-mode contract, the Epic C migration path, and
the leak-cleanup one-liner that targets `LitellmManagedBy=agent-vm-provider`.

LIT-2878

infra/ami/README.md

@@ -0,0 +1,122 @@
+# litellm-agent-runtime AMI
+
+Builds the AMI that runs the per-session agent daemon on EC2. Consumed by
+the EC2 VM provider in
+`litellm/proxy/agent_session_endpoints/vm_providers/ec2.py`.
+
+## Layout
+
+```
+infra/ami/
+├── litellm-agent-runtime.pkr.hcl   # Packer config (HCL2)
+├── scripts/
+│   └── install-runtime.sh          # apt + node24 + python3.13 + uv + bun
+├── files/
+│   ├── litellm-agent-runtime.service  # systemd unit
+│   └── daemon-stub.py              # placeholder daemon (Epic C replaces it)
+└── README.md
+```
+
+## Prerequisites
+
+- Packer 1.10+ (`packer version`)
+- AWS CLI configured with the BYOC profile (default: `litellm-poc`)
+- IAM permissions on the build profile: `ec2:RunInstances`, `ec2:CreateImage`,
+  `ec2:RegisterImage`, `ec2:CreateTags`, `ec2:CreateSnapshot`,
+  `ec2:CreateKeypair`, `ec2:DescribeImages`, `ec2:TerminateInstances`,
+  `ec2:DeleteKeyPair` (the standard Packer `amazon-ebs` set)
+
+## Build
+
+```bash
+cd infra/ami
+packer init litellm-agent-runtime.pkr.hcl
+packer build \
+  -var "aws_profile=litellm-poc" \
+  -var "region=us-west-2" \
+  litellm-agent-runtime.pkr.hcl
+```
+
+Build runs on a `t3.large`. Total wall time on the BYOC PoC account: ~6–8
+minutes (mostly apt + node-source). Cost per build: ~$0.50.
+
+The output prints the new AMI ID, e.g.:
+```
+==> Builds finished. The artifacts of successful builds are:
+--> litellm-agent-runtime.amazon-ebs.litellm-agent-runtime: AMIs were created:
+us-west-2: ami-0abc1234...
+```
+
+Capture this AMI ID into `config.yaml`:
+
+```yaml
+agent_settings:
+  vm_provider: ec2
+  ec2:
+    default_ami_id: ami-0abc1234...
+    default_region: us-west-2
+```
+
+## Sharing the AMI cross-account
+
+The PoC account builds privately. To share with a customer's BYOC account
+without rebuilding:
+
+```bash
+packer build \
+  -var "aws_profile=litellm-poc" \
+  -var 'ami_users=["111122223333"]' \
+  litellm-agent-runtime.pkr.hcl
+```
+
+Packer adds the `ami_users` to the AMI's `LaunchPermissions`.
+
+## Boot modes
+
+The daemon (stub or real) honours `LITELLM_AGENT_MODE` from EC2 user-data:
+
+- `session` — cold-boot path. Daemon hits `/v1/internal/sessions/{sid}/bootstrap`,
+  then heartbeats every 30s.
+- `warm` — warm-pool path. Daemon idles waiting for hydrate (B2). The stub
+  just heartbeats; the real implementation lands in LIT-2890.
+
+Required user-data env (written by the EC2 provider):
+
+```
+LITELLM_SESSION_ID=...
+LITELLM_TEAM_ID=...
+LITELLM_AGENT_ID=...
+LITELLM_BASE_URL=https://your-proxy/
+LITELLM_AGENT_MODE=session
+LITELLM_DAEMON_JWT=<scoped JWT minted by proxy>
+```
+
+The provider writes these to `/etc/litellm-agent/runtime.env` (mode 600) and
+the systemd unit reads them via `EnvironmentFile=`.
+
+## Replacing the daemon stub (Epic C)
+
+The stub at `files/daemon-stub.py` is intentionally minimal. Epic C replaces
+it by:
+
+1. Bumping the Packer config to copy the real daemon entrypoint into
+   `/opt/litellm-agent-runtime/daemon`
+2. Re-running `packer build`
+3. Updating `agent_settings.ec2.default_ami_id` in `config.yaml`
+
+The systemd unit + boot-mode contract stays the same.
+
+## Tags
+
+Every resource Packer creates is tagged `LitellmManagedBy=agent-vm-provider`
+so cleanup tools can find leaked builders. If a build is interrupted and
+leaves an instance behind:
+
+```bash
+aws ec2 describe-instances \
+  --filters Name=tag:LitellmManagedBy,Values=agent-vm-provider \
+            Name=instance-state-name,Values=running \
+  --query 'Reservations[].Instances[].InstanceId' --output text \
+  --profile litellm-poc \
+  | xargs -n1 aws ec2 terminate-instances --profile litellm-poc --instance-ids
+```