refactor(backends): extend NativeToolEngine from ClaudeCodeEngine

[aaight]

Summary

• Migrates ClaudeCodeEngine to extend NativeToolEngine instead of directly implementing AgentEngine
• Implements the three abstract methods: getAllowedEnvExact() returns Claude-specific env vars, getExtraEnvVars() returns { CLAUDE_AGENT_SDK_CLIENT_APP: 'cascade/1.0.0' }, and resolveEngineModel() delegates to resolveClaudeModel()
• Updates afterExecute() to call super.afterExecute() (context file cleanup) then clean up the Claude-specific persisted session directory
• Deprecates the standalone filterProcessEnv wrapper in claude-code/env.ts (kept for backward compat with tests); env filtering is now handled by the base class buildEnv()

Test plan

• All existing claude-code.test.ts tests pass without any changes to assertions (82 tests)
• All existing claude-code-env.test.ts tests pass without changes (15 tests)
• NativeToolEngine.test.ts passes (19 tests)
• Full unit test suite passes (6717 tests across 350 files)
• TypeScript type checking passes with no errors
• Linting passes with no errors

Trello: https://trello.com/c/69c1aa44cc343ba5291ec4dd

🤖 Generated with Claude Code

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

[codecov]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/backends/claude-code/index.ts	72.72%	3 Missing ⚠️

📢 Thoughts on this report? Let us know!

[nhopeatall]

Summary

LGTM — Clean refactoring that correctly migrates ClaudeCodeEngine to extend the NativeToolEngine base class. The three abstract method implementations are consistent with the existing values used by buildClaudeEnv(), afterExecute() correctly chains to super, and the removed supportsAgentType/resolveModel are properly inherited. All CI checks pass.

Should Fix (non-blocking)

• Residual duplication: execute() still calls the standalone buildClaudeEnv() (which hardcodes ALLOWED_ENV_EXACT and the extra env vars) rather than this.buildEnv() from the base class. Both produce identical output today, so this is not a bug — but it means the env-building logic is now duplicated in two places (buildClaudeEnv and the base class buildEnv). A follow-up could replace buildClaudeEnv() usage in execute() with this.buildEnv(...), which would make buildClaudeEnv truly dead code (and the @deprecated annotation fully actionable).

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