refactor(desktop): renderer/main mechanical cleanup batch

Part of #1084 (second-pass sink-file and boundary governance) — the desktop+ui mechanical batch, re-verified against current main. The composer workspace-row extraction from the original batch is already done (packages/ui/src/composer-workspace-row.tsx).

Scope — one PR, all mechanical

• Rename to kebab-case with import/test updates: renderer/OnboardingHero.tsx, renderer/FirstRunChecklist.tsx, renderer/settings/SettingsModal.tsx, renderer/settings/ProvidersPanel.tsx (the last PascalCase holdouts in their directories; note contract tests such as settings-modal-focus-churn-contract.test.ts reference them).
• Move the 4 authored Markdown skill bodies out of main/skills.ts into content files: starterSkillTemplate (~L1446), officeCliDocxSkillTemplate (~L1472), officeCliXlsxSkillTemplate (~L1509), officeCliPptxSkillTemplate (~L1548); keep only the id→body mapping.
• Replace the last hand-rolled pending-set in renderer/app-shell.tsx (addPendingSessionAction ~L475 / clearPendingSessionAction ~L486 over bare Set refs) with the useKeyedPendingRegistry hook already used 6× in the same file.
• Relocate the stray deepResearch:get handler (main/main.ts:867, the only ipcMain.handle left in main.ts) into a register*Ipc module.

Invariants

Same as #1084: behavior- and visually-neutral; flat PR off main.

Verification

Typecheck/build + affected contract tests; visually neutral.