feat(ai): Comprehend PII redaction utility for EdgeChains (#290)

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Summary

Implements #290 — $50 AWS Comprehend PII redaction for the JS SDK.

Deliverables

• `ComprehendPiiRedactor` / `createPiiRedactor` in `arakoodev/src/ai`
  • `redact(text)` and `redactMessages(messages)` for chaining before LLM calls
  • Injectable `detectPii` for tests; default uses `@aws-sdk/client-comprehend` when installed
• Unit tests: `piiRedactor.test.ts`
• Example: `JS/edgechains/examples/comprehend-pii-redact/index.mjs` (runs without AWS keys using a local detector stub)

Note on demo video

Runnable example is included. Loom recording can be added on request if maintainers require it for acceptance.

Test plan

```bash
node JS/edgechains/examples/comprehend-pii-redact/index.mjs

pure redaction logic verified in CI/test suite

```

Fixes #290

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