Resolves #290.

Added AWS Comprehend PII-redaction to the EdgeChains JS SDK — a Comprehend client (axios + built-in-crypto SigV4, no new deps) and a chainable RedactChain (comprehend.pipe(endpoint).chat(...)) that redacts prompts before any Endpoint class observes them, plus jest tests and a runnable example; existing endpoints left intact per the bounty’s “chain with” criterion (loom video placeholder must be filled in manually).

/claim #290