Adds hmacSha256Hex and hmacSha256Base64 to @sim/security/hmac and migrates
15 webhook providers plus 5 other hot paths (deployment token signing,
outbound webhook requests, workspace notification delivery, notification
test route, Shopify OAuth callback) off bare `createHmac` calls. Secret
parameter accepts `string | Buffer` to cover base64-decoded Svix-style
secrets (Resend) and MS Teams' HMAC scheme. AWS SigV4 signing in S3 and
Textract tools intentionally retains direct `createHmac` usage — its
multi-step key derivation chain doesn't fit a generic helper.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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