fix(mcp): cap codegraph_context output to prevent context bloat (#296)

Route handleContext's output through the shared truncateOutput cap
(MAX_OUTPUT_LENGTH) so codegraph_context can no longer blow past the context
budget — every sibling MCP tool already truncates; this was the one uncapped
output path.

Closes #296

Co-authored-by: Baijack-star <71923891+Baijack-star@users.noreply.github.com>