Add cpg_get_last_write MCP tool

[narangtanmay]

Summary

Adds an MCP tool that returns the writes that may reach a given node's value, one-hop, control-flow-sensitive reaching definitions.

• Node.reachingWrites() reads the node's existing prevDFG edges (already materialized by PointsToPass's kill/union analysis) and returns them as ReachingWrite(source, granularity, functionSummary).
• cpg_get_last_write exposes this over MCP: resolves a node by ID, runs reachingWrites(), returns the writing nodes with their dataflow granularity and whether the edge came from a function summary.

This is deliberately different from followPrevDFG: that API returns a single witness path by design (findAllPossiblePaths = false), so on a variable written on multiple branches before a merge it picks one arbitrary write. reachingWrites() returns all of them, the full "may" set.

Test plan

No automated tests included yet, open question whether to add them before merge.

• Manually exercised against a small fixture with branch writes before a merge point, confirmed multiple prevDFG sources come back
• ./gradlew :cpg-core:compileKotlin :cpg-mcp:compileKotlin clean

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Tanmay seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}