Add Fusion ingress/egress policy setup guide

fusion/POLICY_SETUP.md

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+# Fusion policy setup (steps 2, 3-route, 8)
+
+This is the one part that isn't Terraform/CLI — it's configured in the Amplify
+AI Gateway (Fusion) itself. Below is the flow the gateway must implement. It is
+the orchestration + transformation layer only; detection and the vault are the
+separate components it calls.
+
+## Endpoints to fill in
+- `PRESIDIO_URL`  = `http://<presidio-task-ip>:5001/analyze`
+- `VAULT_TABLE`   = `hncb-deid-demo-vault` (DynamoDB)
+- `AGENT_RUNTIME_ARN` = printed by `scripts/agentcore_setup.sh` (step 3)
+
+## Ingress policy  (advisor request → cloud)
+1. **Authenticate** the advisor and apply the RBAC / business-purpose check.
+2. **Detect**: POST the raw query to `PRESIDIO_URL`. You get back typed findings:
+   `[{entity_type, start, end, score, text}]`. (Findings, not redaction.)
+3. **Mint + splice** (Fusion owns this):
+   - for each finding, generate a fresh **random** token — different every request.
+     Convention: `CUST_<random>` for a PERSON that resolves to a customer,
+     `TW_<random>` for a `TW_ROC_ID`, etc.
+   - write the reversible entry to the vault:
+     `{ token, type, value, session_id, expires_at }`
+   - replace each finding's span in the text with its token.
+4. **Route**: invoke `AGENT_RUNTIME_ARN` with `{ "prompt": "<de-identified text>" }`.
+5. **Trace**: log the **tokenized** payload only — never the pre-substitution text.
+
+## Egress policy  (cloud response → advisor)
+1. Receive the agent's de-identified result.
+2. **Restore**: for this `session_id`, look up the vault and re-attach the real
+   identity (envelope-level here — the talking points are generic, so you prepend
+   "王小明 —"; if any token appears inline, swap it back too).
+3. Return `{ "final": "<restored answer>", "deidentified_prompt": "<what the cloud saw>" }`
+   so the UI can show the split view.
+
+## Vault item shape (DynamoDB)
+```
+{ "token": "CUST_000123", "type": "CUSTOMER", "value": "cust-0001",
+  "session_id": "<conv id>", "expires_at": <epoch+ttl> }
+```
+
+## Demo-simplest alternative
+If wiring all of this into Fusion policy is too much for the first recording,
+implement steps 2-5 as a thin Fusion flow that calls a small "tokenizer" Lambda
+(detect → mint → vault write → splice) and a "restore" Lambda on egress. Same
+architecture on screen, less console clicking. The point of the demo is that the
+gateway owns the flow and the cloud only ever sees tokens.