refactor(backends): move contextFiles and nativeToolPrompts to shared module

[aaight]

Summary

• Moves offloadLargeContext, cleanupContextFiles, and buildInlineContextSection from src/backends/claude-code/contextFiles.ts to src/backends/shared/contextFiles.ts
• Moves buildSystemPrompt, buildTaskPrompt, and buildToolGuidance from src/backends/nativeTools.ts to src/backends/shared/nativeToolPrompts.ts
• Keeps backward-compatible re-export shims at all original paths so no engine imports need to change
• Updates the barrel src/backends/contextFiles.ts to re-export from shared
• Updates test import path to point directly at src/backends/shared/contextFiles.ts

Closes: https://trello.com/c/pq3Vm1a6/349-as-a-developer-i-want-context-files-and-prompt-building-moved-to-shared-so-that-all-native-tool-engines-use-common-modules

Test plan

• All 15 existing claude-code-contextFiles.test.ts tests pass
• TypeScript type checking passes (npm run typecheck)
• Lint passes (npm run lint)
• All engine index.ts files compile with no import changes needed (shims ensure backward compat)
• Note: postComment.test.ts, github.test.ts, and misc.test.ts failures are pre-existing on origin/dev and unrelated to this refactor

🤖 Generated with Claude Code

🕵️ claude-code · claude-sonnet-4-6 · run details

[nhopeatall]

LGTM - this refactor cleanly moves the shared prompt/context helpers without changing engine call sites or behavior. The backward-compatible shims remain in place, CI is green, and the backend unit coverage for these prompt builders/context offloading still passes.

🕵️ codex · gpt-5.4 · run details