docs(ghostkey): bring the page in line with what actually shipped

[sanity]

Four claims on /ghostkey were describing an older state of the system. Found while sweeping for staleness after this week's delegate changes.

The developer section was a version behind. It said an app "sends a SignMessage request" — which requires knowing a fingerprint before you can do anything, and is the same version-behind problem the skill and the ghostkeys README had. This is the most-read of the three. Now leads with SignWithDefault (no fingerprint, prompts and replays if the app holds no grant) and mentions HasIdentity, which answers "does this user have a Ghost Key" without prompting — the thing that lets an app decide whether to offer a purchase path before putting a dialog in front of someone.

Bundled issuance was on the roadmap. The page said "the missing piece is bundled issuance", which reads as imminent. It is parked. The page now says why rather than leaving a reader to guess: Ghost Keys are principally a CAPTCHA replacement for one-off actions and already do that job, so per-action unlinkability is an improvement, not a prerequisite.

ZK was "the direction we expect Ghost Keys to move in". Also parked, and we now know considerably more than when that was written. Replaced with the measurements: ~400-byte proofs, ~84 ms to verify inside a contract, and the real obstacle being structural rather than speed — contract state is re-validated on every load, so verification has to happen once at admission, which a contract cannot do alone.

Backup advice predated the vault doing anything about it. It recommended downloading a backup as an aside. The vault now marks an identity un-backed-up and shows a one-click download that clears only on explicit confirmation, so the page frames it as the point of the exercise and says plainly that some nodes reclaim idle storage.

Deliberately unchanged

• The two news posts. Dated announcements with aspirational future-tense, not documentation making claims that are now false.
• The delegate diagram. Its SignMessage label still depicts a real request; rewriting positioned SVG text for a nuance a diagram cannot carry isn't worth the layout risk.
• The create page's "Why the amounts are fixed". It says "a short list" without enumerating tiers, so it survives the ladder change in feat(ghostkey): extend the donation ladder to $500 / $2,500 / $10,000 #100 untouched.

Docs only.

[AI-assisted - Claude]