fix(memory): write graph atomically in saveGraph to prevent file corruption

[AbhiPra24]

Description

Fixes #4614.

In KnowledgeGraphManager.saveGraph() (src/memory/index.ts), the knowledge graph was written directly in-place to this.memoryFilePath via fs.writeFile. If the process was terminated mid-write (SIGKILL, container stop, OOM kill, power loss), the memory file could be truncated or left corrupt without recovery.

This PR makes saving atomic by:

1. Writing the serialized graph content to a uniquely named temporary file (${this.memoryFilePath}.${Date.now()}-${random}.tmp) in the same directory.
2. Atomically renaming the temporary file over the target this.memoryFilePath using fs.rename().
3. Cleaning up the temporary file if an error occurs before renaming.

Changes

• Update KnowledgeGraphManager.saveGraph in src/memory/index.ts to write to temp file then rename.
• Add test in src/memory/__tests__/knowledge-graph.test.ts verifying atomic persistence and no leftover temp files.

Verification

• Ran npm test and npm run build in src/memory. All 51 tests pass.

[Selven81]

Thanks — this is the most tightly scoped of the five PRs on #4614, and
splitting the relation validation out into its own change is what makes both
easier to review.

I also followed the naming convention shared by all four open PRs here: the
temp file is not hidden behind a leading dot. If a save dies badly and leaves
an orphan, it should show up in a plain ls — and the leftover-temp-file checks
written in these PRs scan with readdir by prefix, so a hidden name would make
them blind.

Summary of what I took from each PR, and how I validated the merged version:
#4614 (comment)