Closes two items from the platform/completeness audit appendices.
Amber signature verification (audit "open verification item", LOW):
reconstructSignedEvent() previously trusted whatever `sig` the external
Amber signer app returned — it recomputed the event id but never
verified the Schnorr signature. A compromised or buggy signer could
therefore return a forged/garbage signature that the app would treat as
valid and publish. Now verifyEvent() is run on the reconstructed event
before it is returned; failures return null, which every caller already
handles as an "incomplete signature" error. This single choke point
covers all Amber paths: the native AmberIntent signer, AmberCallbackPage,
and AmberCallbackHandler.
F9 (LOW): pin MCP server versions in .mcp.json. Both js-dev-mcp and
nostrbook/mcp were fetched at @latest, silently pulling the newest
release into the agent tool environment every session. Pinned to the
current exact versions (0.6.3 / 0.5.3) so upgrades are deliberate.
Co-authored-by: shakespeare.diy <assistant@shakespeare.diy>
