fix(graph): preserve matched nodes when source exceeds output budget

[iveteamorim]

What

Fixes mex graph get <id> when a matched node's full source does not fit within the output budget.

Instead of dropping the node and reporting no-match, graph get now:

• preserves the node's compact fact
• returns the largest whole-line source prefix that fits the remaining budget
• reports status: "partial" with truncated: true
• suggests a larger --max-output-tokens value calculated to retrieve the full source

Successful source-backed graph get responses are unchanged.

Why

Fixes #230.

Previously, a node could exist and be matched while its source record exceeded the output budget. The source would be omitted, returnedNodes would fall to zero, and the summary would incorrectly report status: "no-match".

That makes a budget limitation indistinguishable from a missing node.

Type of change

• Bug fix
• Regression tests

Testing

• Added regression coverage for an oversized matched node at the default 1500-token budget
• Verifies a compact fact is returned
• Verifies a truncated source prefix is returned within budget
• Verifies the response is partial, not no-match
• Verifies the suggested retry budget retrieves the full source
• Added coverage for a tight valid output budget
• protocol-v3-golden.test.ts: 4/4 passed
• npm run typecheck: passed
• npm run build: passed
• git diff --check: passed

Existing successful graph get protocol goldens remain unchanged.

[theyashasvipandey]

Thanks @iveteamorim! I pushed a small follow-up commit (every oversized id now keeps its fact, and the retry suggestion keeps --max-source-lines). Merging.