feat(webauthn): passkeys in the vault — dcrypt as a software authenticator

[pyramation]

Summary

A passkey is not a stored password — it is a P-256 keypair, and holding keys in an encrypted store is precisely what a platform authenticator is. So dcrypt becomes one: @decryption/webauthn mints the key, keeps the private half as a concealed vault field, and signs the challenge a site issues.

The signed bytes are the whole point:

signature = ECDSA-P256( authenticatorData || sha256(clientDataJSON) )
clientDataJSON = { type, challenge, origin, crossOrigin: false }
authenticatorData = sha256(rpId) || flags || signCount [|| aaguid || credId || coseKey]

The origin and the RP id hash are inside what is signed, so a site cannot obtain a signature naming a different origin — that is the unphishable property, and it is why nothing here can be replayed elsewhere. verifyAssertion checks the signature the way a relying party would, so the round trip is actually proved rather than asserted.

Attestation is fmt: "none" with an all-zero AAGUID. This is software; claiming hardware attestation is the exact lie attestation exists to catch, and a relying party demanding it should reject us.

PasskeyStore puts the key in the vault as a new passkey item kind, concealing only the private key — a site name and a sign count are not secrets, and leaving them readable is what lets the list render without decrypting anything:

Field
rp_id, credential_id, user_handle, user_name, sign_count	plain
private_key	concealed

assert() persists the advanced sign count before returning, because a site that later sees the count fail to advance is entitled to conclude the key was cloned — losing that write is worse than losing the assertion.

From the CLI:

dcrypt passkey register auth.example.com --user dev@example.com --challenge "$CHALLENGE" --json
dcrypt passkey assert auth.example.com --challenge "$CHALLENGE" --json   # pipes into a relying party
dcrypt passkey list | forget

The challenge is required rather than generated — signing a challenge we invented ourselves proves nothing. It is allowed in argv, unlike a password: it is a public nonce.

Not in this PR: the relying-party half against constructive-db's auth:passkey procedures (webauthn_begin_registration / finish_registration / begin_sign_in / finish_sign_in), and the desktop UI. Those procedures are deliberately crypto-free and are currently tested with opaque-but-plausible bytes, so this is the piece that will let them be exercised with real signatures for the first time.

One infrastructure note: adding a workspace package forces pnpm to re-resolve, which trips minimumReleaseAge on a transitive ws@8.21.3 that the lockfile already pins (published 41h ago, gate is 48h). Rather than weaken the policy, the new importers were added to pnpm-lock.yaml directly — the resolution step is then skipped and the install is a no-op, as on main.

Link to Devin session: https://app.devin.ai/sessions/04636534e07048089ffb6b78142e12cd
Requested by: @pyramation

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