feat(core): improve edit tool output

[rekram1-node]

Summary

• replace the synthetic old/new diff preview with concise replacement-count output
• report the actual match count when an edit is ambiguous
• include the target path in no-match failures
• return specific missing-file and directory errors while preserving existing typed failures

Testing

• bun test test/tool-edit.test.ts (packages/core)
• bun typecheck (packages/core)
• git diff --check