Remove dead code found by full Periphery audit; add scan config + advisory CI (#1410)

Periphery 3.7.4 audit of both schemes (macOS + iOS, intersected so
platform-specific code is never touched), with test targets indexed and
the share extension built. 277 dead declarations removed or demoted:
dead forwarding wrappers (ChatViewModel+Nostr/+PrivateChat), removed-
feature remnants (autocomplete command suggestions, back-swipe tuning,
MediaSendError, GeohashParticipantTracker), unused Tor dormancy
bindings, assign-only properties, unused parameters (renamed to _), and
redundant public accessibility. 13 orphaned localization keys deleted
across all 29 locales (old pre-#1392 location-notes UI, app_info
warnings).

Two real tests were flagged as unused because they never ran: Swift
Testing methods missing @Test (NostrProtocolTests.
testAckRoundTripNIP44V2_Delivered, NotificationStreamAssemblerTests.
testAssemblesCompressedLargeFrame). Re-armed both; they pass.

Deliberately kept, now recorded in .periphery.baseline.json: iOS-only
code invisible to the CI macOS scan, C FFI signatures, keep-alive
NWPathMonitor reference, InboundEventKey.eventID (dedup semantics),
wifiBulk capability bit (reserved for Wi-Fi bulk work, used by
BitFoundation package tests), and the String secureClear cluster
(exercised by package tests).

New: .periphery.yml config and an advisory Dead Code CI job (mirrors
the SwiftLint precedent from #1361) that fails on findings not in the
committed baseline.

Verified: full macOS app suite, BitFoundation (119) and BitLogger (13)
package tests green; periphery scan --strict exits clean.

Co-authored-by: jack <jackjackbits@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

bitchat/Services/NoiseEncryptionService.swift

@@ -153,11 +153,11 @@ enum EncryptionStatus: Equatable {
 final class NoiseEncryptionService {
     // Static identity key (persistent across sessions)
     private let staticIdentityKey: Curve25519.KeyAgreement.PrivateKey
-    public let staticIdentityPublicKey: Curve25519.KeyAgreement.PublicKey
+    let staticIdentityPublicKey: Curve25519.KeyAgreement.PublicKey
     
     // Ed25519 signing key (persistent across sessions)
     private let signingKey: Curve25519.Signing.PrivateKey
-    public let signingPublicKey: Curve25519.Signing.PublicKey
+    let signingPublicKey: Curve25519.Signing.PublicKey
     
     // Session manager
     private let sessionManager: NoiseSessionManager