refactor(architecture): decouple main state bridges and extract store transform helpers

[Totopo27]

Note: This PR builds upon #1083 (security boundaries). Once #1083 lands, this PR will automatically update to show only the architectural refactoring diff.

Summary

Decomposes apps/desktop/electron/main/index.ts by extracting scattered mutable lifecycle bridges into a dedicated MainProcessState container (apps/desktop/electron/main/bootstrap/main-state.ts), reducing the main entrypoint by over 450 lines of repetitive accessor boilerplate while strictly preserving contract semantics. In addition, pure data transforms are decoupled from the central Zustand store (apps/desktop/src/stores/app-store.ts).

Key Changes

1. State Container Pattern (bootstrap/main-state.ts):

   • Encapsulates mutable runtime references (mainWindow, host, sidecar, quitting, closeBehavior, tray, etc.) into a cohesive MainProcessState class.
   • Houses the bridge interfaces consumed by subsystem factories:
     • launcherState: LauncherState
     • windowLifecycleState: WindowLifecycleState
     • runtimeState: RuntimeState
     • applicationLifecycleState: ApplicationLifecycleState
     • applicationAppearanceState: ApplicationAppearanceState
     • planRuntimeState: PlanRuntimeState
     • startupState: StartupState
     • shutdownState: ShutdownState

2. Entrypoint Simplification (index.ts):

   • Replaced more than 350 lines of standalone getter/setter boilerplate across index.ts with unified destructuring from mainState.
   • Entrypoint line count decreased from 1,484 lines down to 1,029 lines (~31% reduction), bringing it closer to repository architectural ratchet bounds (AGENTS.md §7).

3. Workspace Git Resolution Extraction (workspace-git.ts):

   • Extracted withGitBranch into a dedicated workspace utility module.

4. Error Formatting Centralization (main-process-errors.ts):

   • Consolidated describeError into the error-handling module alongside installMainProcessErrorHandlers.

5. Store Decoupling (stores/helpers/store-helpers.ts & stores/app-store.ts):

   • Extracted 13 pure workspace and session data transformations into store-helpers.ts, dropping app-store.ts from 736 to 617 lines (~16% reduction), complying with AGENTS.md §8 ("Pure transformation → Reducer / helper").

Verification

• Executed full test runner across Electron Main, BrowserView, and UI Store suites with 54/54 passing tests.