Support signing with hsm

[wagoodman]

This is not complete and needs a lot of work and verification.

Adds an AWS KMS signing path so the Developer ID private key never has to live on the signing machine. The existing --p12 flow is untouched.

What's in the branch:

• --kms-key and --kms-cert-chain flags on quill sign (and sign-and-notarize). URI scheme matches sigstore/cosign (awskms:///alias/... or awskms:///arn:...).
• New quill csr subcommand for enrolling a Developer ID cert against a KMS-resident keypair (Apple's normal CSR flow assumes the private key is local... with the key in KMS that path doesn't work).
• KMS-backed crypto.Signer implementation pinned to RSASSA_PKCS1_V1_5_SHA_256. We explicitly do not use sigstore/sigstore/pkg/signature/kms because it forces RSA-PSS, which Apple Developer ID CMS does not use.
• TestSign_KMS runs the existing trusted fixtures through an in-process fake KMS provider and asserts byte-identical CodeDirectory + CMS output to the P12 path. Passes codesign --verify.
• .github/workflows/apple-e2e-kms.yaml + workflow_dispatch-only pipeline that signs quill itself with a real KMS key + real Developer ID cert and submits to Apple Notary. The only thing that proves Apple actually accepts what we produce.
• docs/hsm-signing.md: threat model, end-to-end walkthrough, IAM policy, troubleshooting.

TODOs before the Apple E2E workflow can run (one-time, manual):

• Create an RSA_2048 SIGN_VERIFY KMS key + alias (alias/quill-ci-signing or similar).
• Create an IAM role with kms:Sign + kms:GetPublicKey on that key, OIDC trust scoped to this repo + main.
• Run quill csr against the new key, submit to Apple Developer portal, assemble chain.pem from the issued cert + Apple intermediates.
• Set repo Variables (non-secret, cert chain is public material): QUILL_KMS_KEY_URI, QUILL_KMS_CERT_CHAIN_PEM, AWS_KMS_SIGNING_ROLE_ARN, AWS_KMS_REGION.
• (Existing) APPLE_NOTARY_ISSUER, APPLE_NOTARY_KEY_ID, APPLE_NOTARY_KEY secrets reused as-is.
• Dispatch the workflow once to confirm the wiring end-to-end.

Out of scope, follow-up issues:

• GCP KMS (gcpkms://) and Azure Key Vault (azurekms://) providers.
• PKCS#11 provider (CloudHSM, YubiHSM, on-prem).
• ECDSA Developer ID support.

Closes #759