fix(server): thread graph_rel_types through the mcp kb_context tool

[Yurii214]

summary

the mcp kb_context tool exposes the graph-expansion knobs expand_graph / graph_depth / graph_limit but drops graph_rel_types, which the jsonl _h_context handler threads and build_context_pack fully supports (context.py → graph.graph_neighbors_for_seeds(..., rel_types=...)).

so an mcp agent — claude code / cursor / codex, the primary way vouch is consumed — can graph-expand a context pack and tune its depth/breadth, but cannot restrict which relation types the walk follows. expand_graph=true pulls neighbors across every relation type (contradicts, supersedes, references, …) with no way to scope to, e.g., only references. a jsonl/http /rpc client can. the sibling kb_neighbors tool already exposes rel_types on mcp, so relation-type filtering is a first-class mcp capability everywhere except context-pack graph expansion.

this is the same surface-drift class as the kb_export exclude fix: pr #185 added graph_rel_types to build_context_pack and the jsonl handler but its mcp-tool hunk never threaded it.

fix

add the optional graph_rel_types: list[str] | None param to kb_context and thread it into build_context_pack, mirroring _h_context.

test

test_kb_context_mcp_honors_graph_rel_types (in tests/test_graph.py) builds a claim with a references edge to auth and a blocks edge to risk, then calls the mcp tool with graph_rel_types=["references"] and asserts auth is included and risk is filtered out. it fails on the current code (TypeError: unexpected keyword argument 'graph_rel_types') and passes with the fix.

validation

mypy src (117 files, no issues), ruff check on the touched files, and the graph / context / capabilities / server-tool-surface / jsonl-server-surface suites all pass locally.

[github-actions]

diff coverage: n/a — this PR changes no python under src/vouch/, so there is nothing for the gate to measure.