Clear the Minor findings from the hardening review

[foodlbs]

Summary

Clears the 11 Minor findings logged during the hardening review (PR #3). No behavior changes; three test gaps closed. Follow-up to #3.

Code

• SkipperApp — url is a non-null constant, so the if (url != null) guard was dead; removed and the block dedented. The six catchNonCancelling sites that swallowed silently (onFailure = {}) now log at Log.d, so a failed cached-config read/parse or version read is diagnosable instead of vanishing.
• RemoteConfigParser — dropped the redundant .take(256) on customButtonList's viewIds/labels. Verified first: stringList already caps every entry at MAX_MATCHER_STRING (added in Hardening & refactor: 1-month roadmap from ownership audit #3), so the second cap was a no-op. The cap itself is unchanged.
• proguard-rules.pro — corrected two misleading comments and removed the inert -keep class java.security.** rule. Those are Android platform classes, never part of the app's dex, so R8 could never strip them; the rule implied a protection that didn't exist. The DataStore comment claimed R8 "can inline and rename" key strings — R8 never rewrites string literals; the keep actually guards against class shrinkage, which the comment now says.
• SkipperKitSettingsScreen — alphabetized the semantics imports.
• TeachScanner — KDoc explaining that packageName is intentionally unread (the throttle is global, matching the single timestamp it replaced).
• DiscoveryCoordinator — removed the injectable classifier param and its imports. Nothing injected it; DiscoveryEngine.discover already defaults it (YAGNI).
• SECURITY.md — note that the key-relocation mv must run from the repo root (it uses a relative source path).

Tests (3 gaps closed)

• TeachScannerTest — a clickable node with no id/text/desc (and a blank-after-trim one) must be filtered out. Previously only the isClickable half of the predicate was exercised, so a regression stripping the isNullOrBlank checks would have gone unnoticed.
• SkipEngineTest — NeedsGesture.displayLabel on the CUSTOM path. The existing custom-button test always hit the Clicked path, leaving the gesture-fallback label untested.
• DiscoveryRepositoryTest — addApproved on a pending key (promotes and clears from pending) and on a dismissed key (promotes). This edge changed behavior in Hardening & refactor: 1-month roadmap from ownership audit #3's lifecycle unification and was flagged as untested.

Test plan

• ./gradlew testDebugUnitTest lintDebug assembleDebug assembleRelease — BUILD SUCCESSFUL
• assembleRelease included deliberately, since ProGuard rules changed

Not included

The Critical host-hygiene items (C-1 signing key, C-2 release keystore) are maintainer actions on local key material, not code — runbooks are in SECURITY.md. Still outstanding, along with the R8 runtime smoke test and TalkBack check.

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