lore-overlay: cross-platform MVP — capture on macOS + Windows with both signing pipelines

[tcballard]

Sub-issue of #321 (lore-overlay, ADR-093). Depends on #322 (extraction).

Corpus: Initiative 1 of rac/roadmaps/lore-overlay.md; platform posture ratified in ADR-100 (cross-platform from the outset — macOS + Windows first-class, Linux deferred).

Scope:

• Tauri v2 tray app: global hotkey → non-activating modal → the rac-capture interview → draft PR via the GitHub App → the two-gate model (ADR-077; the app's identity can never approve/merge, ADR-065).
• Both platforms in the initial release cycle: macOS (Developer ID signing + notarization) and Windows (Shell_NotifyIcon tray, RegisterHotKey, WS_EX_TOPMOST; Authenticode / Azure Trusted Signing, SmartScreen-reputation ramp, bundled/bootstrapped WebView2).
• Settings surface (gateway endpoint/key/model; target repo + GitHub App; hotkey) — populated by admin provisioning (lore-overlay: admin-provisioned setup — one-time configuration and provisioning docs #324), not by the author.
• Success bar: an author on either platform produces a schema-valid artifact (rac validate exit 0) from the hotkey interview, writing no Markdown, landed as a draft PR promoted only by an independent merge.
• Known least-charted part: desktop GitHub-App device-flow auth + on-device token caching — treat as this initiative's spike.

Tick on #321 when the success bar is met on both platforms with signed builds.