Improve resume-related actions for session search results

[chumyin]

Summary

• add a source-aware Copy Command action for resumable search-result sessions
• show Resume in CLI for Codex and other resumable sources, not just Claude
• resolve a safer working directory for resume commands, with validation and fallback behavior
• add Codex fixtures and regression coverage for the new resume-related actions

Testing

• packages/core/node_modules/.bin/vitest run packages/app/src/shared/resumeCommand.test.ts packages/app/src/main/sessionResume.test.ts
• npm_config_cache=/tmp/spool-npm-cache PNPM_HOME=/tmp/spool-pnpm-home PNPM_STORE_DIR=/tmp/spool-pnpm-store npx pnpm --filter @spool/app build:electron
• node_modules/.bin/playwright test --config e2e/playwright.config.ts e2e/fast-search.spec.ts

Closes #37